Going through this same thing now, using a '95 Riv for my Donor. From my research, the Driver's side Manual Fiero, with the tulip changed with the Donor Tulip, should be used on the driver's side (Not the passenger side). Length is 22 5/8" on stock Fiero Axle for reference. On the Passenger side, the length needed is 25 1/16". On this side, my research shows using the Fiero outer CV, the Inner Tulip from your RIV should work fine, but you need an axle shaft to mate these two up. Most have used one from the passenger side of an '89 Pontiac 6000, non heavy duty. The Cardone PN is 60-1115. At least that's the way I've got it figured out so far- haven't actually gotten to the point of trying it yet.
Here's some links from my research:http://www.fiero.nl/forum/A...041015-1-041467.htmlhttp://www.fiero.nl/forum/A...041015-2-046145.html
'87 GT in process!