AFAIK the Scooby STI R180 diffs just bolt up with no problems, but you need to swap over the front flange (that bolts to the pinion) from the Nissan diff. Also the STI side axles aren't compatible with the early Nissan driveshafts (with the bolt in the middle), so you need some sort of adapter axle like this one, which clips into the Scooby diff, and the flange is Nissan-friendly: http://www.betamotorsports.com/products ... php?cat=11
Subaru R180s seem to come in 3.9, 4.1 and 4.4 ratios, and depending on age, will be a TrueTrac, Torsen or 2-way plate centre.
The other requirement which I think might be required, is that the Beta side axles use the modern-style spline, where they slide into the diff and spring-clip into place (as opposed to the KGC driveshafts which bolt to the diff centre). At full-droop, the KGC driveshafts are at maximum extension, so I think if you wanted to use the clip-in style of side axle, you would need to use a shorter rear shock, or come up with some sort of droop limiter to prevent the driveshafts from pulling out of the STI diff. I'm not sure how much of an issue this would be in practice tho
I've got a 4.4 STI plate diff and a pair of the Beta Motorsports axles in the garage, but been too lazy to fit them, so I can't vouch for how it all works out
Alternatively, OS Giken does a Super Lock LSD centre for the old-style R180s, but it's pricey
No Kev, you are eating a duck fetus.