It sounds like you are running out of camshaft at 2000 rpm if the acceleration dies of at that level. Can't see a camshaft in a Pontiac doing that though unless the lobes are down. That tranny must have pretty close gears to only have a 1000 rpm drop between first and second , second and third. I wouldn't be afraid to shift at 4000-45000 and see what your performance is there. You may be able to accelerate through the bog. When you had the camshaft out did you notice any stampings on the snout of the shaft? A letter stamped there could tell you what the camshaft is.

IF you are getting a good acceleration from stand still and then bogging at 2000 I wouldn't think it's due to the rear gear. If you had a low ratio gear it would bog right from the start and pick up a bit as speed increased. If you had a really high ratio gear you would get good initial acceleration right up to the point your engine stopped making more HP and your rpm would increase rapidly. Definitely check the ratio though as Cal says. You could spin the rear wheel 10 times while counting driveshaft revolutions to get a more accurate reading.

You could check the distributor's mechanical advance weights to be sure they are free, maybe they are not moving out as speed increases thus not giving you more advance with more speed. Also check the initial timing and the vacuum advance. And definitely make sure the secondaries are opening. Did you have that carb apart to clean and/or rebuild when you did the engine?

Only other thing that jumps out at me is fuel delivery, maybe running out of fuel when you go through 2000 rpm. Filter, pump, hose collapsing?