Robby:
A 4 jaw "independent" chuck- that is, a 4 jaw chuck with each jaw independently adjustable with its own screw- does not have "inside" or "outside" jaws. The jaws are reversible. Just don't mix them up. There will be numbers stamped on the chuck body (1,2, 3, 4), and there may be the same numbers on the jaws. While it might not matter if the jaws are mixed up, I like to keep them in their original tee slots. They may have worn in, even if they were made to a close tolerance when the chuck was new.
When you remove the jaws from a 4 jaw independent chuck, the jaw screws often want to fall out of the chuck body as they are running in thrust blocks that only engage 180 degrees of the circumference of the shouldered area on each screw. No harm done if the screws fall out, it is an opportunity to clean them.
As for removing the chuck: As Ramsay notes, LeBlond used a very coarse spindle thread ( 2 1/8"-5). I do not know what methods you have tried to break loose the chuck thus far. With that coarse spindle nose thread, it has been my experience the chucks and faceplates break loose of the spindle thread fairly easily. When I was buying my 13" Roundhead Regal lathe, it had sat un-used in an unheated building for about 12 years. The previous owner/user had died 12 years prior and his son, while knowing machine tools, did not know the finer points or history of the LeBlond lathe in his late father's shop. Since there was no power turned on, I rolled the lathe by hand in each gear to be sure no gears were damaged in the headstock. After that, I wanted to be sure I was not getting a lathe with a chuck locked onto the spindle threads. I took a short mechanic's pinch bar, perhaps 18" long, and put it across the chuck jaws as a lever. I put the lathe in its lowest speed, and gave a good sharp yank on the bar. The chuck broke loose so easily I was surprised.
Here is a thought for you: if the chuck does not break loose of the spindle threads by "reasonable means of persuasion", I'd suggest trying the following:
1. dismount the chuck from the back plate. This will leave the backplate screwed onto the spindle, but the end of the threads will be more accessable than if the chuck were still mounted.
2. Make up a steel bar to use as a lever. This bar should be made from something like 3/8" x 2" wide A36 (basic hot rolled structural steel) flat bar. Drill the bar so you can bolt it to the backplate. The bar does not need to be too long, maybe 18" beyond the outer circumference of the back plate.
3.Bolt your "cheater" to the backplate and make sure the bolts are made up solidly. You can drill two sets of bolt holes: one set will allow you to mount the bar so it spans the back plate diametrally, the other set should span two adjacent holes, forming a "chord" (remember your HS geometry ?).
4. Start with the cheater bar bolted to form a chord, since this will give access to the end of the threads. Cut a piece of sheet steel so it has a "mouse hole" to drop over the hub of the backplate between the backplate and the headstock. Make this sheet steel plate large enough to protect the headstock. It is going to be a heat and flame shield. With the heat shield in place, apply heat to the backplate, putting some of the heat on the hub. A smaller and concentrated torch flame helps here. Using a torch such as a "Mapp" torch or "B tank plumber's torch" or a brazing tip on an oxyacetylene torch, apply heat to the backplate, putting heat on the hub, and some on the face of the backplate, staying back from the hole with the spindle threads.
5. Get the backplate "too hot to touch", but not any much more. Take a dead blow hammer and strike a few blows on the "cheater bar" to try to break the backplate loose.
6. If the backplate does not break loose, warm it back up and spray on some penetrating oil. "Kroil", if you can get some, is my preferred penetrating oil. Another trick that old timers used on "frozen" threads, particularly on steam locomotive and marine steam plant work, was to get the entire area with the frozen threads hot, then apply beeswax. The beeswax, when melted, forms a thin oil and this "wicks" into the threads. As the parts cool, a partial vacuum (at least in theory) is formed in any clearance spaces in the mating threads, and either the penetrating oil or the beeswax is likely to be drawn in. As this is happening, take about a 2 lb steel hammer and start striking LIGHT blows on the cheater bar. The impact and the expansion due to the heating, along with (hopefully) some lubrication may do the trick.
7. If this method fails, see if you can get some dry ice pellets. Some welding supply stores will sell dry ice pellets. This is a convenient way to handle dry ice.
If your local supply sells dry ice pellets, bring a cooler and get a few pounds of them. Put a wad of rags in the spindle bore, pushed in a bit further than the length of the spindle nose threads and shoulder. Pack the spindle tapered bore and a few inches beyond into the straight bore of the spindle, with the dry ice pellets. Plug the open end of the spindle taper with a loosely wadded rag to keep the dry ice pellets from falling out. The loosely wadded rag will let any CO 2 gas given off as the dry ice "thaws" escape freely.
8. After you get the dry ice in place in the spindle, repeat the heating on the backplate. The idea here is to create enough of a temperature differential, as well as enough "movement" due to expansion/contraction, to break the bind in the threads.
We used the dry ice trick many time with certain frozen parts in powerplant work. It has been my experience that sharp blows, as from a bronze or steel hammer, often will break things loose even when what seemed like heavy torque applied via wrenches or similar means would not.
9. Another trick is to combine torque + impact. To do this, you may be able to pull it off it you are strong and reasonably coordinated. If not, get a helper. Put a pipe cheater on your "cheater bar" that you bolted to the backplate. A piece of pipe that adds perhaps 24" is about all the long I'd go to avoid building too much torque thru the gearing. Hold a good strain on the cheater pipe with one hand. Using a 2 or 3 lb steel hammer while strain is held in the cheater pipe, hit a series of "Light to Middling" blows with the hammer on the flatbar bolted to the backplate. Hitting on the pipe will do little, if any, good. Holding strain on the pipe takes the "bounce" out of things and puts a torque into the spindle nose threads. Adding a bit of impact often does the trick. Again, unless you have a good "feel" for this kind of work, it is all too easy to wreck the gearing. Light snappy blows with a 2 lb steel hammer combined with what I call a "good strain" on the cheater pipe should do the trick. Add heat to the backplate and dry ice in the spindle bore if this last method does not do the trick by itself.
As an aside: If I had only ONE chuck to have on my lathe, it would be the 4 jaw independent chuck. This type of chuck lets you chuck odd shaped jobs, turn eccentric work, and lets you adjust each jaw so a job runs dead true. A 3 jaw "universal" or "scroll" chuck is a convenience, but it is never something you want to rely upon to get a job running dead true with an existing diameter (such as turning a journal on an existing shaft or boring a part so the bore is concentric with the existing outer diameter). Damned near anyone can set up a job in a 3 jaw chuck so long as it is round, or so long as the number of sides (such as a hexagon) is divisible by 3. If you had to chuck a piece of square or rectangular stock to turn a cylindrical section, you'd be SOL with a 3 jaw chuck. I keep the 4 jaw chucks on my lathes and only put the 3 jaw chucks on for occasional jobs. Get the hang of "bucking in" work in a 4 jaw chuck so it is reasonably true within about 0.015" total indicated runout- which you will develop the ability to see by eye without using a dial indicator, you then use the dial indicator to get the work running dead true or as close to it as you can get it (as you will discover, what looks round and is supposed to be round, in actuality may be out of round by a few thousandths). As you develop some experience using the 4 jaw chuck, your speed and ability to center up a job in it will increase as will your confidence. The 4 jaw chuck has the advantage also of being able to tighten each jaw individually with a jaw screw. This puts more clamping force into the work than the 3 jaw scroll chuck develops. This is a good thing when you get into heavier jobs- who needs a job slipping in the chuck jaws and getting chewed up by them ? Another advantage to the 4 jaw chuck is that you can put pieces of soft copper between the jaws and finished work (such as a shaft journal), then buck the job into true, copper shims and all.
I have often said that if I were aboard a ship with only one chuck for the lathe, or in some isolated place with only once chuck for the lathe, it would be a 4 jaw independent chuck.
I am guessing that someone may have run the chuck onto the spindle of your lathe and not entirely "bottomed" the backplate hub against the shoulder of the spindle. They then proceeded to start the lathe at a fairly high speed and nailed into a cut. This "wound up the chuck" on the spindle under power and with some impact. Some persuasion of a like nature, helped along with some heat (or cold) and penetrating oil should do the trick. When you dismount the chuck from the backplate, make some "match marks" so you can put it back in the same position relative to the backplate. I use light prick punch marks, since Sharpy or paint stick marks tend to wear off during the kind of work you will be doing to get that backplate broke loose.