This is just some stuff to think about...
Your cam has about the same specs as a Performer Camshaft. This is intended more towards low to mid-range torque. The single plane intake isn't helping lowend performance matters. Same issue with the double pumper carb. A dual plane intake and vacuum secondary carb would be better matched to your cam. What distributer are you running? I hope it isn't a computer controlled version.
What tranny do you have? The stock 200-4R?
I wouldn't be surprised to see that your rear gears are around 2:56's... A boggy motor will have lots of issues spinnin those tall gears...
Mark