Sounds like a fun venture in interchangability.
If you can fold your own receiver with extra length, then more power to you.
My initial impression on using 8mm in a stamped config, was that it had to be
the 1.6mm thickness.
I picked up a ColdSteel heavy yugo blank with extra length and converted a
Romy RPK kit into an 8mm rig. The recoil did hammer the rear trunnion loose
from the rivets. Some rework of rivets and welding the trunnion fast solved the
I found the 80% blank worked well because you can field all your layouts for the specific caliber.
Dimensionally, it is similar to the PSL receiver, because the cartridge lengths
are fairly close. I would not be afraid to convert a PSL receiver to 8mm. I think
the rail lengths are about correct, the magwell would need reworked.
If I remember right, the Mauser chamber threads would need to be turned down
and sleeved to get to the correct width for M70 trunnion. Otherwise, with some
mods, it's doable.
Go with the PSL bolt and carrier. The bolt face would need a sleeve to reduce diameter.
BTW, Green Mountain makes new M76 barrels.
I've tried one, and it's at least as good as the originals, if not better on accuracy.