As said on the Z garage website about a T5 swap I raided an 82 ZXT for it's differential flange. However it doesn't even fit on my stock differential! It won't slide on and the spline opening is larger than the stock flange! What's the big idea??

The ZX flange isn't going to work on the stock R-180 if that is what you are trying to do. If you want to use the ZX driveshaft, you could have a good driveline shop swap swap over the front U-joint over onto the stock 240 driveshaft, might be a bit expensive since they'd have to cut the old one off, weld on the ZX piece and then re-balance the entire driveshaft.