Thank for info and link mate
One guy here have done some research about those Kia diffs too. I do know that some diff goes to Fb, but I think that it wasnt straight swap. It costs about 250€ new.
That\'s a good price considering that racing beat http://www.racingbeat.com/RX7-1975-1985/Ring-Pinion-Differential.html
Charge a lot more.
Get into ausrotary and post in that thread I linked you too, the guy will sort you out with the right info. My slip diff cost £650 delivered to my door iirc