Regarding the weight, I know that Philly Ordnance offers an option on their M1928 dummy receivers of milling a full-length channel on the inside so as to mimic the original weight (still won't accept the bolt). Maybe they can do the same for an M1.
As to this idea of "80% completion," it's basically meaningless. Nobody can define what "80% completion" really is, and it's not mentioned anywhere in the ATF regs. It's just a shorthand way of referring to supposedly acceptable dummy receivers. Philly Ordnance has ATF letters approving their specific products, so you're OK as long as you don't make any further changes. The only "rules" as to what constitutes "constructive possession" of a machine gun come from some old court cases, which seem to imply that a dummy receiver can fall into that trap if it can be turned into a working machine gun with 8 hours or less of work in a well-equipped machine shop.
Finally, regarding those neatly saw-cut demilled receivers that turn up occasionally, they're the result of old import regs that allowed a couple of saw cuts. Now, a receiver has to be torch-cut, with the cuts displacing at least 1/4" of material. If you have a parts set, and all the portions of a saw-cut receiver, it would be fairly easy for the ATF to make a case of "constructive possession" against you (after all, all it would take would be a few welds to restore the gun to operable condition, and the ATF doesn't care what it looks like as long as they can get it to fire two shots with one pull of the trigger). Certainly, welding a saw-cut receiver would take less than 8 hours in a well-equipped shop. Bottom line: be very, very careful messing with those saw-cut receivers.
