The 7075 aluminum are a no go for welding. It will break at a later date. The 6061 can be welded then machined and should survive.
I’m surprised there isn't an 80% forum group, but have been struggling to see the benefit of an 80% build.
I get the incognito aspect, but if I were to be involved in a self-defense shooting incident, I would think that the guy carrying the gun without a serial number is not going to be cast in the most favorable light.
The cost savings doesn’t really seem to be there - 80percentarms.com (for example) is selling lowers for $80, but (whenever they’re back in stock), RK has had stripped lowers for $50, I see Spikes forged lowers elsewhere for $93. With that jig at $300, is there really a cost benefit? Then there’s the resale as well - I don't buy with the intent to resell after x years, but there’s certainly value to what I have in the safe. If I fell ill and felt that my 80% build “gun” maybe a burden for my family, would a dealer even take it on consignment?
I get the tinkering/pride of a personal build aspect, and I considered building a P220, but unless I build 10 of them, I just don’t see it happening from a cost perspective.