Ok, i bought those same rear sheet metal stampings from brp. I found that i actually had to have a gap on the bottom between those 2 halves to be at the correct width when welded. I determined the correct width by placing the buffer in between the halves, then i added a .013 shim to one side for clearance since the welds would shrink it. I took pics but haven't posted yet. Also of note, i made the relief cuts for the buffer to rotate properly in both halves before i welded them together. I also put as much of the buffer as i could in the front half to tack it together as well. It also helped square it up. After i was done welding,(i did NOT weld the whole thing cause most of it will be cut out anyways) my buffer was to tight so i put the receiver in a vise and gave it a little squeeze to widen it out a little. It worked great! In hind-site i should have used a slightly larger shim but.....That was my experience. If the stampings are consistent i would say DO NOT make them any skinnier! They need to be almost 3/32 wider. But ya, metal needs to be remover all over the place!
Measure, think about it, measure again, think about it some more, measure again, then cut......."Oh S***!!!!"