My 454 powered dually runs a 4.10 gear, 32.25" tires and an overdrive ratio of .75. At 2500, I am moving along at 76MPH. That tank is mucho heavier than your Imperial and has absolutely no problem keeping up with in any situation. I believe that your 440 is a more effecient engine than the big block Chevy also giving it an edge. I would go to a 2.76 gear with an RV cam and some 27"-28" tires. The cam and small valves is the key to port velocity.
Just my $.02 worth.