What type of rear end came in a 1991 with a 305?

Can they be built to handle any hp?

If not, what other rear ends are a good swap in?

IROC?

Unless your swapping out the 305, a stock G80/G92 rear should hold up fine. These are the RPO codes for the performance rearend that had posi for 3rd gens. I've seen guys with 383 L98 strokers with the TPI setup with stock rears. Not sure of HP, but probably over 400. You can find one at a salvage yard cheap or add teh posi parts to yours. Other than that, I would think you would be better off going the 4th gen direction than 2nd/1st gen. This is only is you want to stay cheap - if you want new, then anything goes.

-Dan