How many miles does the '84 motor have. Pulling around that heavy truck can really work a slant over good after 20 years. Have you done a compression test? I don't recall seeing of differences in the crank used in cars vers trucks after they went to the cast crank, but I'm no expert as you know.
I expect from seeing your collection that you've given the duster a good tune-up by now. Timing, cap, rotor, wires, air filter. Check the Heat riser operation.
If you're comparing a super six motor with 2.76 gears to a 1bbl motor with 3.23 gears, I think the super will still feel like a slug at a stoplight.
You could have the head shaved to increase compression and get a look at the condition of the valves/ bores while you're in there.
The newer motor has a hydrolic cam. You could have a problem with the lifters or cam. I don't know what to tell you on that. The '84 cam specs ARE different from the '74 motor. The anti pollution cam grind might be a performance killer?
