I had a set of 2.56 gears in my '64 Tempest, they came in it from the factory along with the 215-cube six and 2-speed auto. I kept them when I transplanted a 455/TH400 combo into it, absolutely wonderful on the highway. Here in So Cal if you can't maintain at least 75 to 80 MPH on the freeways for long distances you'll get run over.
After a few years the 2.56 gears gave up and I swapped in some 2.93 gears. Not long after the swap I was longing for those 2.56 gears.
My brother has a '69 350 Firebird that came with 2.78 gears. We swapped in a warmed-up 350 HO engine and changed to a 3.42 rear from a second-gen T/A. He said it was barely liveable on the highway.
In my opinion if the car is going to see any open-road miles 3.08s would be the lowest I'd consider. I would consider 3.23s to be right on the borderline, you'll definitely hate the 3.55s.
