[pat in Santa Cruz]

check timing for sure, as too retarded will rob top end power.. Its not an uncommon situation. Your leaky manifold will decrease power too. Check it when you get the new one and be sure both manifolds are flat and with the ports in a straight line. You might have to have them surfaced. Check that your brakes are not dragging. A compression check never hurts, and it will give you a starting indication of the internal condition.

Once you get it running right, you can make it stronger with high compression head, cam, better breathing.

Once you get it up and running strong, you can always get a Mitchell Overdrive for it. I know someone around here with a 31 AA flatbed that goes 70.