MFer. I just posted a long explanation and the server choked.

Get the 97+ Q45 diff and see if the output shafts are 30-spline. Just because they have the "J30" bolt pattern for the axles (as you describe) doesn't mean anything. Late JDM S14 and S15 have 30-spline outputs and 3x2 flanges. If they do have a 30, pop your output shafts in. Done.

Or, get an R33 GT-R diff. Bushing mounted housing, 6x1 flanges, 4.11 gear, and clutch/mechanical center.

(by the way, 30-spline is the big spline that's on Q45 and Z32TT among others. Baby spline is 29-spline from the S13/S14/Z32 NA/100 others)