If you have the option, leave a little material on all the surfaces but the bore (sized for broaching), cut the splines, then somehow put a mating male spline "mimic" accurately trued in a lathe, then mount the new broached flange on the male spline and finish turn all accessible diameters and faces. Done right, should give the best perpendicularity, concentricity, and balance when you're finished.
Have done some this way but the expanding splined arbor can be $$$$ ... did quite a bit of vertical broaching when I worked at Websterville, and generally we didn't need to go to that extreme.
First thing with ALL gears is to do the bore and one face at the same time. KEEP TRACK ! This is the biggest problem with cutting teeth-only for people, they usually aren't real careful about this but it's like Rule Number One in cutting gears - bore truly square to one face. Then that face is your reference for all tooth cutting.
The second is, if they are flat gears and going to be stacked for tooth cutting ? PARALLELISM ! You can put four or five blanks on a mandrel you think is strong - inch, inch anda half, even 2" - tighten the nut and bend the mandrel .025" or .030" easy. Hard to believe but put an indicator on it ...
Then on a vertical broach, there's a bunch of play, we didn't try to lock everything down tight because the tension on the broach will pull everything straight. There's a groove on the bottom end of the broach that is gripped loosely by a collar, you'd ease in on the hydraulic cylinder to take a little tension first then let 'er rip.
The grip on the broach was fairly loose so it could self-center and pull straight. And the part was free to float around on the plate for the same reason. It was just a flat piece of steel with a hole in it, nothing special. We had different drop-in anvils for different size holes.
It worked pretty good, probaly not
as good as finish turning off the spline but it was good enough for Hewland gears. Most important was cleaning the plate the part sat on so chips didn't make the part sit crooked. And the gullets under the teeth. One of my co-workers pulled a row of teeth off and collected his check that afternoon, bye bye. Made an impression, can you tell ?
There was a hose with like a garden nozzle for slooshing oil all over the broach before pulling it. Also got all over yourself. As the broach pulls through, every row of teeth makes a little corona of oil, sploosh sploosh sploosh. And you can't really stand far away or you couldn't reach the parts and broach and handles to operate the machine ... wear rubber underwear, after a few hours of this one is a sulfurated mess.
Horizontals take up a lot more floor space (air is free - at least so far) and the broach has a tendency to sag, so you need a follower. We'd frequently pull about 3' long broaches, the operator platform was maybe three steps up, made for a tall machine but small footprint. For the price they get for broaching machines I'd probably build my own too