Interesting conversion. This looks like a semi trailing arm solution lifted from one of the many Japanese sports cars (due to the 4.5" bolt circle). Among the many it could be from:
Third generation Toyota Celica GT-S A60 Series (1982-1985) and or its related Supra variant
Nissan 200SX, 240SX 300ZX, 200SX, original 510, 610 and a few others
After looking around it appears to be a Subaru rear diff and rear suspension.
It is also similar to the E21 BMW rear end but BMW never put the spring perches behind the arm, always on top and the CVs are not like the European style.
This pic is of a Subie rear diff:

This is I believe an R160 but I am not entirely sure.
So do you have more pics or part numbers on any of the parts that might help ID the origin of this mod?
How does it drive? My experience with semi trailing arms is you'll need to watch the lift throttle oversteer. Interesting mod in any case.