need oomph in the 30-60mph range

The trans being worn out is dragging you down IMO, I would check wheel and axle bearings and u-joints for drag, sounds like your timing could be off a bit.
I had a recurved distributor built by a member here and I have a bunch more low and mid range and I can feel it in the seat of the pants H.P.
A good tune up does wonders, Is this a hydraulic lifter /6 or does the valves need to be adjusted ?
I have seen a couple thread on the /6 engine is not strong enough here lately, I guess my 2.740 lb Valiant must be a good match for one.