What gear ratio are you running? Once the engine gets out of the torque band where it's able to accelerate, it will lug down. It's either time to downshift and let the engine rev, or go to a somewhat lower rear end gear. Most stovebolters get a case of the vapors anytime an engine gets much above fast idle, so we've got a bunch of lugged-down engines running around at 40 MPH in either high gear and/or overdrive with a 3-something rear end ratio. The truck doesn't have enough torque to get out of its own way like that. Drop a gear, get the engine up around 3K RPM, and watch it scoot when you push the loud pedal! Back in the late 1960's my bone-stock 58 Chevy Delray coupe with a 235, a 3 speed stick shift and a 3.55 rear axle had no problem running 75-80 MPH, and that was on fairly low profile tires on 14" wheels. The engine rarely saw less than 3K RPM.
Jerry
