ok im new to this site and to the third gen cars....
i got an 88 gta firebird... i had a new sbc 355 installed with all the stock tpi equipment... the car has tons of torque and power but the rear gear ratio is holding me back.....
i was told this particular gta came with an australian rear which there is no aftermarket support for... So is there a rear out of another car or model that i could use as a base to build or direct swap?????

 

Count the bolts on the diff cover. If it has 9 bolts, it's a Borg Warner diff and parts are hard to find. A factory 10 bolt is weak but a fair amount of parts are available. If you're lucky, you might find a bolt in Dana 44 but chances on find one are rare.

That leaves aftermarket options. A few vendors produce a bolt in 9", 12 bolt or Dana 60. Expect to pay at least $2500 for a complete performance diff.

Only a diff from a third or fourth gen will be a direct fit and the fourth gen diffs are about 2" wider per side so the factory wheels will stick out. Because of the torque arm suspension, no other diff can be used without changing the complete rear suspension.