Spent a full day at TechShop milling, grinding, and most especially filing. It looks like all the parts from the parts kit will more-or-less fit:
The shroud retaining lever fits, but the shaft was sheered near where the nut would be. Right now it is a tight friction fit but if I want to be really nice, I need to cut off the shaft, press-fit a new one, and thread that for a new retaining nut.
The magazine catch release appears to fit in the existing slot, with the pin being press-fit. At the very least I have to re-drill the holes, as one got slagged over.
The charging handle appears to be assembled through the gun, with the bolt catcher also pinned on. Sounds very fiddly to do! (I don't even know how you can press the -- missing -- pin in once the side rails are welded on).
I just figured it out. There's a hole in the side plates that is drilled slightly larger than the retaining pin. Trouble is, this requires moving the bolt back slightly from out of battery -- something no longer possible since I welded it in. But I should be able to just mill the slot a bit further and make it work.
The sight was wrenched off but even straightened, the remains of the rivets do not appear to be located properly. So I'll need to grind those off, possibly re-mill the slot a little to achieve a proper fit, then drill new holes. Which I might want to tap, or use soft brass on, so the next person along doesn't have to lever the sight assembly off.
The trigger assembly is held at the front by a block with a notch in it; only the stub of this block remains as the rest was in the direct path of the cut. So I have a few hours yet -- among other things, cleaning off the cosmoleum or whatever it is on the remaining original parts so they fit and move properly.
On the receiver itself: it is never going to look pristine, and I spent all morning with diamond bit and small hand files just re-shaping the barrel lugs after my last attempt to fill some of the remaining voids. But at the same time, it is starting to look decent and it might just be worth trying to fill the two worst gaps; the remaining slot in the side plates, and the small gaps to either side of the end cap. The latter are visible when the weapon is assembled.
To do the latter, I'll want to lathe up a new backing plug from the last of my 1-1/4" aluminium rod. Although since the bolt is now a permanent part of the rebuilt receiver, there's no good reason to keep any more of the rear clear than is needed for the rod that is part of the end cap. The big downside is I'm sure to damage the threads again, and have to grind and file those some more.
The side plates are a bit more troublesome. I had a long and difficult time trimming weld metal that made it past the end of my last set of backing plates, so I'm leery of welding near the tube. But pretty much, would be milling a chunk of aluminium plate to stand in for the trigger group. And probably milling into the existing side plates to provide a slot for fresh pieces of steel. The MIG will fill a lot, but even with a backing plate, where I fill is where I'm going to have to go back with mill and grinding bits to clean out the slot again.
I'm reading up on prep for gun bluing now. I have both brown and blue and should be able to get a nice aged patina out of them. I suspect very strongly I'm going to have to spend 10-20 hours continuing to file things smoother and smoother, then graduate to finer and finer emery papers, then finally to steel wool and wire brushes. So there's a bit of labor involved, still!