If it were my truck (I do love me some old IH trucks) it would get a Cummins 4bt motor with the P pump so fast it wouldn't be funny. That would be killer! Can make 300+ whp fairly simple and get great fuel mileage compared to the gasser, not to mention TORQUE! probably make around 5-600 ft/lbs

I'm a Cummins 12v enthusiast so that's my first choice of truck motor