Man if your gonna do a new engine and your obviously a gear head I say the 440. How fun and cool would that be? EVERYONE has a 318 or 360- kind of a yawn. I say build a torque monster 440 with a nice dual exhaust and good dual plane manifold with a great street carburetor like the poly Street Demon (Modern upgraded Thermoquad). Building a torque monster means utilizing OEM parts like rocker arms and heads- this keeps cost down and preserves reliability. Having 480 plus lb feet of torque at your gas pedal would make driving your work truck fun. Run the tried and true ( and affordable!) Trans Go shift kit in a robust 727 transmission. Yea gas mileage will be not good but your not gonna get great mileage with any engine you presented. Might as well have fun with that big wide legendary 440 Mopar!!!!!