Mike, Glad to see you using the site...
Ok, so first thing is if your NOT going to be do any track time the 8.2 10 bolt freshen up with new bearings and posi etc will more than do the job on the street and even some trac duty as long as the car does not get the clutch dumped above 3500 rpms with slicks on a sticky track.
-10 bolt 8.5 is an excellent choice and will also give you that almost same look as a tempest rear...If your trying to keep it looking stock.
-10 bolt 8.5 uses the same size pinion shaft as the 12 bolt Chevy diff which is what makes it one of the strongest GM rears for cars and light trucks.
-10 bolt 8.5 also has the desirable BOLT IN axle system as your Pontiac rear (28 spline stock can be upgraded with MoSER axles to 30 spline WE STOCK THESE... )giving you the confidence that if a axle does snap the axle will be retained in the housing and NOT ruin a 1/4 panel.
-10 bolt 8.5 has 10 bolts holding the ring gear to the carrier unit (larger 7/16" x 20 thread count per inch)
-12 bolt chevy is again a great choice...getting more expensive and harder to find in unmolested conditions.
-12 bolt has a slightly bigger ring gear at 8.875 which will keep the heat down on long hauling and freeway driving.
-12 bolt has an extra 2 bolts holding the ring gear steady too (3/8 x 24thread count) smaller than the 8.5
-12 bolt has the 30 spline axles and held in with c-clips (can be bad news if the axle breaks because the axle will slide out of the housing and tear up a rear 1/4 panel and maybe cause a accident too.)
So all in all the 8.5 10 bolt will be cheaper to build and will give you good strength for the street and strip applications
The 12 bolt will be more expensive and give you good resale value to your car and of course be strong.
10 bolt 8.2 - we can get you 3.36 3.55 3.73 3.90 gears and posi units to build up that rear also if you really do not think your ever going to need the strip strength. I have had GUYs test the 10 bolt 8.2 with 455 Buick and Pontiacs even at the track with NO issues. 12.0 cars that were mostly street bound but have had the slicks on to race.
Ultimately it is your choice on the rear and what you think you will need. We can guide you the best we can with the info given.
Ask yourself what you will be doing mostly with the car...you should not be too far in wish mode when answering this question but be realistic in the time spent RACING...lol. Everyone thinks they will race ever weekend but end up only getting there a few times in the cars life.
I hope some of this info has helped and keep the questions coming if you need more help figuring the right rear to choose. Jim