There are 3 different carriers for the Pontiac 8.2" 10-bolt.
2-series: 2.56 & 2.78
3-series: 2.93, 3.08 & 3.23
4-series: 3.36, 3.55, 3.90 & 4.33
These are all factory ratios, the only ones available new aftermarket are 3.55 and higher numerically.
A stock 350 might benefit from the 2.56 posi if it currently seems to be spinning the passenger side rear tire on take off with the stock open differential (one-legger). However once you're moving the benefit is gone, no acceleration gain.
My brother had a stock 255HP 2-barrel 350 in his '69 Firebird (a 2-1/4" dual exhaust was the only change from stock), had a 2.78 open rear. We took it to the drags with both a 2.78 and a 3.55 rear. Surprisingly enough there was no gain in ET with the 3.55s, the car ran in the 16.70 to 16.80 range with either rear gear.
However once we swapped in a better cam (factory 068) and 4-barrel intake (stock '69 intake with a '72 750 Q-jet carb) it woke-up and ran between 14.80-14.90.
A swap to better gear ratio might help, might not with a bone-stock engine. If you do decide to swap gears go for 3.08 or 3.23 at the most.
You'll probably hate anything but the 2.56 gears on the highway now that you're used to them.
