I have a 91 RS automatic with a 305 TBI and I'm not sure what rear I have or what gears would be in it. I was told by the guy i bought it from that it has 308's. I also want to know if a posi rear from a 92 GTA will bolt right in cause I might be getting one really cheap.

any rear from a 3rd or 4th gen will be a direct bolt in. pull the cover and look is the only 100% accurate method to know what you have now.

You should have a 2.73 open rear. Normaly all automatic RS's come with 2.73 open and I think (not sure) all manuel RS's came with 3.08 open rears.