If you are looking for an 8.5 look under a GN, T-Type, or 442 ( good luck finding one still in a car in the yard though as they are well sought after ) and be prepared to fork over upward of $1k for a USED rear.
Otherwise a Monte SS has the 7.5 rear and is your next best option for One Stop shopping as it will have good gearing ( 3.42 in the 84 and older and 3.73 in the 85 & up cars ). POSI / Limited Slip was an OPTION so check it before you pull the rear. These are typically $100-$300 used and $1k fully rebuilt. They are fine for a street car but have been known to grenade when using slicks.
F-body and S10 - 7.5 guts will work but the housing wont.