Thanks for the feedback!
'Plug and play' is definitely the goal here. Re-engineer as little as possible. Good news is that the STI diffs are getting cheaper. They've gone down a couple hundred from when I sourced one for my 510 a few years ago. I'd run the same setup for my 280z but I'm working on a potential R200 solution instead.
If I understand your question, the OEM STI inner housing is not swap-compatible with the axle I source. I tried several ideas with no luck.
The R180 STI stubs are welded onto the CV housing in 3 different locations. One weld along the entire perimeter of the stub's flange, three .4" plug welds through the flange into recessed pockets within the housing, and finally one large plug weld on the underside of the flange through the center of the housing. It's not coming off.