I see that you show different carrier breaks for posi and standard. Now I have been building 10 bolt 8.2 B O P and chevy and 12 bolt olds and chevy 12 bolts for years...I have NEVER seen any carrier break difference from any of these from standard(open / one wheel peel) to Posi (limited slip diff units)

From size and make rear like buick early to buick mid to buick 8.5 yes...But never from open to posi.

2.78 ratio will always fit a open or a posi...There is NO posi gear and or standard gear. Period.

As ratios for your Firebirds we DO NOW have these options available...3.36 3.55 3.73 3.90 4.10 ALL NEW and only 219.00 here through your site.

I also can get you posi unit 399.00 NOT much better but still money is money..

rebuild kit 79.00 again bearings are just expensive no matter what you do.

AXLE bearing kit will depend on whether you have sealed early bearings OR later internally oiled taper bearing. usually 69.00 to do both side with seals included.

As far as setting up the rear...It takes patients and does take understanding in how and what you are trying to do then you will be able to do what you need to. The only special tools you will need are a Magnetic base dial indicator And set-up bearing which can be made fairly easy with some power tools. You will need access to a 12 ton press for the pinion bearing and axle bearings for install and removal.

I am always here to help through the questions you might have about this sketchy subject of rear diff set-up. I do not expect people to have all the shop tools so I can help with back yard set-up pretty well.