I really like that work mounting block. What would the machine be called that used the magwell broach?
Broach Press, or broach puller, depending on how it's set or meant to be. I watched a side cutting hydraulic broaching machine cut the 'Christmas tree' ends on turbine engine blades, and the broach was about 20 feet long. I suspect it was rather pricey!
Have seen one rig that was built by a gun maker that was based around a length of all-thread, turned by his bench top lathe, which pulled his broach through the receiver casting or forging. Not too complicated to build, just a couple guide rails and a plate drawn along as required.
There was a mag well broaching machine for AR wells, posted for sale over on practicalmachinist a short while back. Looked like a long stroke hydraulic ram and more guide rails etc., to keep all in line.
A bit overkill for a once or twice ever kind of job, though.
If one were really headed in the direction of broaching out a 1911, or any other forging or casting that required it, it might be worth looking at the available square and keyway style broaches, which could get you there, though more fiddly than would be acceptable if production was the key aspiration.
Think along the lines of building a bushing to fit the milled out mag well, with the ability to slide, say, a keyway broach in to the bushing, to simply clear out the remaining material. If you look at the way keyways are broached, it should give you a pretty good idea of how it's done.
Again, not a great system from a production standpoint, but most home shop gun makers do not have the need to have one separate machine for each operation. Must make do, and trade some time against production volume, eh?
Think a lot of after the war surplus tooling ended up that way, the lathes were certainly WWll vintage. 
