Does anyone know when exactly Pontiac started using the stronger 9" Borg Warner rear, and when they quit using it?
Also, how hard are these to find, if one want's to put one on a later year 3rd generation 'Bird.
Thanks

They started in 85 and ended in 89. They're common enough, but parts for them aren't.

Is there a difference enough to trouble putting one on a 91 GTA? Or is it easier to just get a new aftermarket kit?

It depends what your intentions are.

Street use, maybe everyday. I'd like to get about 350-375 Hp out to thru the rear, good for scaring mustangs and such. I also intend to drop in a 93-97 T56 6 speed, which I figured would help on the highway.
Not going to punish or beat on the car, just want the fire there when I want it, while being streetable.
Did they make that rear in a 3.43?

2.77, 3.27, and 3.45 were the ratios from GM. 3.70 is the aftermarket ratio.

3.70 was a GM ratio as well, but AFAIK EXTREMELY EXTREMELY RARE; it only came in the 85 (and if there were any, maybe the 86) L69 Pontiacs.

They were standard on the GTAs. Both of my GTAs are 3.27. All 9-bolts were limited slip & disc. Stronger than a 10-bolt, but weaker than a 9". For 350hp, you'd be fine with a 9-bolt.