I would follow the suggestions above and definitely figure out what you rear axle ratio is first. If your original rear axle is still in the car, you can try and locate the two letter code that should be stamped into the axle on the drivers side axle tube near the welded on bracket for the brake lines. Or you could get all the factory documentation on your car from Pontiac Historical Services.

Here's the Firebird rear axle chart:

Firebird Rear Axles

Strange things can happen over the now 50 year history of these cars. My California spec 400 coupe was totally intact including the smog devices yet somebody in its past had swapped out my entire factory 3.08 rear axle for a 2.56 freeway gear one.

I don't want to be a stick in the mud, but there is also the possibility that somebody put a Chevy style camshaft in your Pontiac engine (because they are a Chevy enthusiast or used a machine shop well versed in Chevy engines). Pontiac V8's were designed for lower red line RPM's and low end torque. Chevy engine cams lean toward higher RPM's and a torque curve higher in the RPM band.

You could also have a totally mismatched aftermarket intake manifold that is designed for higher RPM's. That will make your Pontiac motor pretty weak lower in the RPM band.

There is the possibility that the person you bought the engine from was getting rid of it because it acted as you describe.