If it were my car, I'd ditch the 307 and find a bigger small block Olds to drop in it since that keeps things cheap and easy. You could even go with a 455 and it still isn't a bad swap. Either way, you're probably looking at a trans change since it most likely has a THM-200 that wouldn't live long.
If you go the Chevy route everything has to change from the Olds setup. As for an SS rear end, nothing special about them except better gearing. The best factory set-up is the 8.5" from a GN, 442 or T-type but be prepared to pay for one. The money starts flying quick when you want to build something to go fast.