Hmm you bring up some interesting thoughts, I was thinking the same as you that there's no need for a full gun in fact just a trunnion barrel bolt carrier fcg and like a tapco flat.
I was thinking maybe a .243 might be better for tested and if possible load in steps all the way to 125% unless of course we have failure before, you're right as we could test with x39 but while we could get some high pressure I think it would be easier to get very high bolt thrust levels without worrying as much about case failure..
I like your yugo barrel idea as it would kill 2 birds with one stone like you said. Also if the thin yugo barrel romy trunnion and bolt are able to fair well with some serious proof loads then I think it would give more confidence in caliber conversions. The math is great but some things just need to also be proven in real tests as well. I also might be able to dig out a yugo bolt and trunnion for testing as well