Your best bet for a 3.73 posi rear is out of an 85-88 Monte Carlo SS. Cost is normally 200-300 bucks for a drum to drump setup. Do not buy it unless you can see inside it before purchase. Also, if the rear is out of an 85', it will have standard studs NOT metric like your 86', so your gonna have to get some standard lugs for the rear, or you will ruin the old lugs or worse the studs. If you want to upgrade, look for rears out of Grand Nationals, T Types, Olds 442s, they came with 3.42s or 3.73s, both are 8.5" rears which can hold up to more power. Much more expensive, I see them range 600-1100 bucks for a unit with original internals. The 7.5" is good until you start gaining traction with big tires and 400+ horsepower. Fine for a street application and a mild engine.