The 50-70's have been built on H & R Huntsman muzzleloader barrels, they have a groove dia. of about .510 and are probably the easiest, and lowest cost platform for building a 50-70 barrel for a handi.
The older barrels with the 7/8" dia. breechplug dont need much work to do the conversion. After you remove the breechplug you need to create a clean flush face for the new breechplug to mate to.
A 25/32 counterbore with a 1/2" pilot does the job of creating a smooth machined surface for the new plug to mate to. A new breechplug is turned on the lathe with 7/8 -20 thread a .780 dia. front sholder and a .890 rear sholder and bored large enough to allow the reamer pilot to insert into the barrels bore.
The new plug should be made of a good steel , 1" dia. 4140 barstock is a good choice and all turning and boring and threading should be done while chucked in the lathe so there is no runout.
The new breechplug should be cut off long so as to give you a surface to tighten it into the barrel,
Once you have a good fitting breechplug with threads that match everything is cleaned well and the new breechplug is epoxy glued and tightened into the barrel. To clean up the excess beechplug material I use a endmill and mill off the excess to within about .010 of the barrels breech surface and deburr the bored hole for turning.
The methoods that I use could be done many other ways, I then turn a piece of roundstock in the 3 jaw so it is running true to the spindle and turn a slight taper to match the bore I made in the breechplug , this will drive the barrel. I made a muzzle plug that fits into the Huntsmans barrel it has a 60 degree center that was bored while the plug was turned.
Now with the barrel being held between centers I use a tool ground to face off any excess material of the breechplug and to insure the barrels breechface is true with the bore when done.
The chamber is then reamed , Other than turning and treading of the new breechplug, I beleive everything else could be done without machines, by someone thats patient and good with a file.
As for the extraction of the fired case, I am just getting to that task but my idea now is to use a H & R ejector from a shotgun and modify it to be a spring powered extractor. It wouold have about 3/16" of travel when the gun is broke open a stout spring would push the ejector out until it was stopped by a pin and be held under tension when the gun is closed.
Wih this methood the barrel could be chambered to any of the longer 50's an work fine.
Jedman