While there are issues (mechanical and legal) with building up a semi auto Thompson, it's not like you are doing anything groundbreaking. My own personal experience with welding demilled Thompsons back together only goes back maybe 40-50 years but have never had one crack. BARs that were of the cast variety yes, they will crack. I remember Jack Tomlin selling cut up Thompson demils by the pick up truck load, Curtis Earl selling the so called demilled ones from the Navy, The sheared ones from that ship in San Pedro and does anyone think that tons of those did not "live again".... Outside of a STEN putting a Thompson back together is about the easiest project in this hobby. And those of you who used to go to Great Western and were into FA, there were stacks of saw-cut two-piece Thompsons, MG34s, UD42s and others sitting on tables for 2 and 3 hundred bucks. Enjoy the things you can do with your own hands, but for god's sake don't be like the person that finds a buried jar of gold coins and has to talk about it, spend it carefully and enjoy it privately....