Detroit engines are noisy, heavy, smoky, and stinky. They also have a very narrow usable RPM band, so they need a LOT of gears. Even a 4-53 will weigh about twice as much as the original engine. You will need to install a front axle designed to hold the weight of the engine and completely re-engineer the steering system, or put the cab onto a frame designed for the engine. Either approach will cause your checkbook to need CPR. I'd suggest retrofitting the truck with a 292 Chevy 6 cylinder and a Turbo 400 transmission. You'll have more than twice the power of the original 216 engine, and other than fabricating new engine mounts, it should be a pretty simple swap. While you're swapping components, install a GM corporate 14 bolt rear end and you'll have a complete new running gear with affordable, easily obtainable parts, not a Franken-truck with a totally obsolete set of drivetrain components.
Edit: If you're really committed to using a 4 cylinder Diesel, drop the cab onto a chassis for a Nissan 4 cylinder Diesel cabover truck and use the whole frame and running gear. It's engineered to work, and you'll have a modern suspension and steering system and brakes.
Jerry
