With the 4X3 transmission setup, you've got enough gears to use any of several engines. I'd suggest one of the big block V-8's if one is available. You won't have to split as many gears with a high-torque engine like a 454 or 502. Just be sure the transmissions and driveline setup is rated for the torque you'll have. Nothing gets expensive as quick as a scattered truck transmission, and one missed shift is all it takes when the engine has more torque than the trans can handle. If you upgrade to a more powerful engine, remember to increase the size of the radiator accordingly.
Jerry
