Thanks for the well thought out post. I can see your concerns, but I feel like the magwell doesn't add to the structural integrity of the frame. I'm a lowly electrical engineer, so metallurgy or structures is certainly not my fortay. I do feel like the forces that are put on the frame are limited to the pin holes and the rear of the receiver.
You are, of course, entitled to your opinion, but think about this, the bottom of the channel is what holds the two sides vertical. Without the bottom of the receiver channel, at the point of the magwell, the receiver is basically two vertical strips of sheetmetal. At 1,400 or 1,600 RPM with 350 ft/lbs of energy at the muzzle per round. That is a lot of energy going into the receiver. Why does the muzzle rise? Because the force is not held squarely behind the action. So there is rotation. There would tend to be compression at the top and tension at the bottom of the receiver. With a sheetmetal magwell welded to the magwell hole, the welded on magwell is holding the bottom of the receiver from pulling apart or buckling. Without the magwell structure welded in, the center of the receiver is basically two vertical strips of sheetmetal.
As far as structural training, I have an associate degree in C.E. & Architectural Design that I received in 1986 and never did anything with. So I have next to nothing to claim that I have structural education. But it would seem to me that the welded in magwell would add structure to the receiver. So removing the welded in magwell for a magwell that is quick change would tend to weaken the receiver in the magwell area. That is just my opinion. Such a project seems like a great one for an 07/02 manufacturer using a post sample machinegun receiver. It would be possible to use a semi for fitting purposes, but I don't think it would be possible to test the stress a machinegun receiver would experience using a semi auto receiver. A machinegun receiver experiences so much more stress because the is such an incredible release of energy. At $8,000.00 + market value, I don't think using a transferable for testing would be prudent. Certainly your gun, yourchoice. YMMV.