Looking at some 17x9.5 rims for my 91' RS, but I'm pretty sure ET 20 won't be enough offset. I've got an LS1 rear end in the car at the moment which definitely changes things. Was wondering if anyone else has got rims like this to fit and what spacers I'd need to run if so, as well as the tire size I should run.

I'm assuming ET 20 is German for 20mm offset? Assuming that is correct, I'd think you'd want closer to 50mm for a 4th gen rear, though I haven't checked specifically lately. Just going off memory. That means with only 20mm, you'll stick out too far. Spacers won't do any good as they'd only push it out even farther. You want to bring it IN....and unfortunately there's no way to change that unless you get new wheels or a new rear.