Turn the air cleaner top upside down so more fresh air can enter-------check the advance mech in the distributor to make sure advance is working-on several older GM dist I have found that the shafts were bound up and advance wouldn't retard so on tune up the timing would get set back and then even with advance it wouldn't be correct.

Are you 100% positive that you have a 3.21 rear gear???????
back in those days there were air dams made to mount on top of cabs to help air flow over trailers

If you have a q jet carb, I would replace it (and manifold) for a 850 or 900 cfm Holley------yes I know all the formulas for carb sizes will disagree-------but, it will give you a better mid range airflow without getting into the seconaries which would then get you into an over rich af and lose power-----------

3.21 gears got to go--------find a 3.73 or 4.11

move out of the hills??????????