[slowmover]

The engine is likely closer to 250HP in actual trim. The "true" numbers don't matter much as it is a torque motor, in main. Find the correct HP/TQ graph (year of production) to see where the TQ peak is located. The old rule-of-thumb from the 1960's & '70's was that cruising rpm should be just below peak torque. Fewer throttle changes this way (rolling terrain) and the engine tends to run coolest, overall. Axle gearing (expensive) and tire height (rolling height) are the places to concentrate so as to get best bang for buck from tires.

EDIT: the ACCEL coil is likely not optimal for the low rpm engine you are running. I'd dump it in preference for what MSD offers (if asked with information at hand about rig). Even a stock DELCO piece would be better. And the QuadraJet can be modded for part-throttle adjustments that are crucial, IMO, for highway economy in heavy vehicles. And using an adjustable kickdown for that TH425 trans is a good idea (dial it in).