Levelling an old and used lathe is always a subject of conversation and debate. The first thing to consider is the levelling depends upon the floor being solid and not subject to movement (such as deflection or "bounce" when live loads such as movement of people, hand trucks or pallet jacks). Next, bear in mind that an old lathe that has been used and moved and setup in a new location is likely to have some twist to the bed. This is a small amount of flexure of the bed and is normal. It takes time to "work out" once the lathe is setup in its new location. For this reason, simply levelling a lathe after it has been moved, let alone with some pre-existing wear on the bedways, will not automatically insure the lathe will turn a "true cylinder".
When a used lathe is re-set in a new location, it often takes a few re-levellings before the bed "settles in" and the twist is worked out of it.
As for levelling: there are several ways to go about it. Hardwood wedges are what the old texts will often show. What I use is shims cut from sheet metal or thin steel plate. I raise the machine tool base using what are known as either "flanging wedges" or "setup wedges"- forged and hardened steel wedges which can be driven with a steel hammer under the base of machinery to get it started lifting off the floor when no provision for using a pinch bar or jack is provided. I then use a pinch bar (length dependent on load to be be lifted) to pick up the machine base. Working alone, I come down on the bar with one hand (or foot if body weight is needed) and with my free hand or free foot, I nudge a thicker piece of steel plate under the machine base. This lets me add or take out shims without danger of things coming down on my fingers. This is a "safety" blocking and is a MUST if you put your hands or fingers under machine bases that are temporarily raised up for adjusting shimming. I cut the shims from anything handy in the way of steel- sheet metal is up to about 10 gauge or 1/8", and 1/8" up is considered as plate. I also keep some rolls of shim stock (stainless steel and brass) in the shop in thicknesses from 0.001" up to 0.030". This can be cut with snips.
I do a trial shimming to get machinery levelled, and then look at the shim "packs" under each foot or mounting location. If there are numerous thinner shims, this tends to create a "leaf spring" effect. I sum up the shims in that sort of pack and then reduce it to one or two thick shims and only 2 or 3 of the thin shims.
Where a machine is to be bolted to the floor, I make what I call "C" shims. These are shims with cuts to the bolt holes in from the edges. These cuts let the shims slip on or off the bolts without having to raise the machine up off the anchor bolts. You can buy these "C" shims ready made, but they can be prices. I make my own.
When making any shim, take care to remove all burrs. On thin shim stock, I take a piece of round stock like drill rod and lay the shim on a piece of flat steel plate. I then kind of wipe the drill rod (maybe 1/2" or 3/4" rod) across the shim to work down any raised areas from the snips, as well as peening the edges with a light hammer to make sure no little "hangnails" from the snips' cuts are sticking up. Any little burr or raised surface on a shim can mess things up and create a condition known as a "soft foot". You think you have your machine levelled, but once loads and vibration and time do their work, the machine goes out of level as the little folds and burrs work down on the shims.
Do not be afraid or ashamed to use whatever is at hand for shim material. Mike any steel that is handy and use it if it is the right thickness. Shims can be cut from the sides of olive oil cans or welding rod cans (done this MANY times), or from galvanized steel used for ducting, or from any thing that seems to fill the bill. Use steel or stainless steel shims under a lathe's base. If you are sitting the lathe on a concrete floor with some un-even or rough surface to it, you can cut the first shim from copper and fully anneal it. This shim will then deform or mold itself to the little irregularities in the concrete. Lay the steel shims on the copper shim. The copper shim is "sacrificial" and its ductility is what works in its favor. If the steel shims were laid right on rough concrete, they might initially bear on some small random high spots. In setting heavier machinery, we always chipped off the surface of the concrete floor or foundation with an air hammer and exposed fresh aggregate. We then set "sub sole plates" on a bed of non-shrink grout, levelling them with a precision level and checking their elevations from one plate to the others. This gave us a solid starting point with complete bearing contact into the underlying concrete. For your little Southbend lathe, a copper shim is plenty good.
Another couple of ideas: you can buy jacking screws with swivel (ball joint) base pads and use them in the mounting bolt holes in the mounting flange on the lathe base casting and in the "pads" or "feet on the tailstock-end leg casting. These swivel pads will conform to irregularities or dips-and-dives in a concrete floor's surface. I use this sort of setup under my lighter Southbend lathe, and put 1/4" pressure treated plywood shims between the pads on the levelling bolts and the floor.
If you choose not to buy pre-made jacking screws with the swivel base pads, you can make something similar. Get some pieces of steel plate about 4" x 4" x at least 3/8" thick. Take a piece of bar stock large enough to accomodate the size of anchor bolt the mounting holes in your lathe's base. Example: if the lathe has 5/8" holes in the base for anchor bolts, get a piece of 1 1/4 or 1 1/2" round stock (I use hot rolled A-36, common structural steel, nothing fancy needed for this). Cut off chunks about 1" long, chuck them in your lathe and face both ends square, then drill thru and tap 5/8-11. Locate the center of each of the 4 x 4 plates and weld these tapped slugs to each plate in the center. Chuck the welded assemblies in the lathe and take a light facing cut across the plate. You can turn a little relief or "pocket" in the face of the plate to allow the plate to seat better on the floor. Welding might have drawn the plate further out of flat.
Use some 5/8" B-7 grade all thread rod for studs and make some washers from 1/4" steel plate. Raise your lathe and set it up on wood blocking temporarily. Slide the plates (with the tapped slugs) under the mounting bolt locations on the base flange and leg pads. Slip an all thread stud down each mounting bolt hole, and thru a plate steel washer (big enough to get good bearing contact on the underside of the lathe's base flange or leg pads), and run the stud thru one regular hex nut and one thin "check nut". Screw the stud into the tapped slug on the plate, and make up the check nut. This locks the stud into the tapping on the plate. Run the hex hut up until it bears solidly on the plate washer. This will be used to jack your lathe up enough to get the wood blocking out, and to level your lathe. Run another hex nut down on the stud so it just is snug against the top surface of the base flange or leg pads. You can put 1/4" pressure treated plywood under the pad plates if your floor is a bit rough. The plywood will compress and conform to the irregularities in the floor and being pressure treated, is less likely to rot out if you wash down your shop floor. I paint these pieces of plywood with a couple of coats oil-based enamel.
Home made jacking "feet" will work like a champ for your lathe. Unless the lathe is driven from a countershaft mounted on the ceiling of your shop or on a wall, there really is no need to bolt it solidly to the floor. using home made jacking feet will get you off the hook for shimming and works nicely if you are levelling your lathe alone. You will not have to jack the lathe up to adjust shims, and with a couple of wrenches, you can tweak the lathe to level.
Again, bear in mind that you can level an old lathe so your precision level shows it as being "dead nuts" (perfectly levelled). However, when you go to take a cut on a long piece of work, you may be surprised to find the lathe still cuts a bit of a taper. This is due to the inevitable wear and twist in old lathe beds. I find that turning work between centers on long jobs, I have to tweak the tailstock offset now and then to get a true cylinder over a long run on old lathes. Levelling, despite what texts and manuals may tell you, is a theoretical sort of thing. In actuality, it is a starting point when you are dealing with an old lathe that has seen use, wear, and been thru a move or two or three.
I do not advise anchoring a lathe solidly to the floor. If you do put in anchor bolts, run them down so they are just snug. A slight pull on the wrench beyond hand tight is all that is needed. Pulling a lathe down solidly by drawing the anchor bolts down hard can give a false levelling or take a lathe bed off level and put a new twist into it. A very light tightening of the bolts is all this is needed.