I just got a rear end from a '88 or '89 IROC to go under my '92 RS. The RS has a 2.73 gear, open differential, and drum brakes. The rear from the IROC has disc brakes, posi differential, and I need to know how to figure out what gears are in it....also, if it ends up havin 3.23s, I want to swap in a set of 3.73s, how much of a performance gain should I expect to see going from the open diff. - 2.73 gears to the posi w/ 3.73s?