Engine Issues
Will the engine fit, or will you have to make firewall or frame alterations? Make sure to factor in height as well. Is the engine too heavy for the suspension? Half-ton stuff is probably OK, but if you try to swap a 4BT into a Chevette, you might be in for some trouble.
Transmission Options
You'll be a step ahead if you figure out what transmission you'll be using. Dodge overdrive transmissions such as the 47RH, 47RE, and 48RE are a good choice, but they're heavy. If you're building a smaller vehicle, adapters are available for TH400s, 727 Torqueflites, and Powerglides, as well as manual transmissions. These transmissions are physically smaller, lighter, and have been proven to withstand up to 800 horsepower if built correctly. Keep in mind that your engine will only spin up to 3,000-4,000 rpm, so if you decide do go with a non-overdrive transmission, you may be gearing-limited. Do some research and find out which transmission makes the most sense for you.
The Vehicle
Make sure your swap vehicle is up for the task of handling the torque of a big Cummins. This means making sure your frame won't twist, and your suspension and axles will handle the power without breaking. The last thing you want to do after successfully completing an engine swap is break everything else.
Finishing Details
You're almost there, but there are still a lot of things to think about. Will the vehicle have an intercooler? Will the steering clear? Can a vacuum pump and stock brake system be used, or will the vehicle have to be switched to a power steering pump-driven hydroboost? Is there enough space between the engine and the firewall to mount the exhaust? Remember, the vehicle will also need a large radiator if it is to be used for towing, and you'll need throttle, transmission kickdown, and shutoff linkages to complete the swap. The best place to find all of these items is from a wrecked truck or on the Internet, as buying all the needed parts from your local dealer could nickel-and-dime you to death.
Swap Time
If you've gotten to the point where you know the engine will fit, nothing will break, and you've decided on a transmission, it's time to be realistic about your fabrication skills and motivation level. The last thing you want to do is have a half-finished project sitting in your garage forever. Engine swaps will most likely require some level of fabrication so be prepared to either do it yourself, or have enough money on hand to pay a shop to do some of the work. If you think you've considered everything on this list, then have at it! The sky is the limit on what type of diesel-powered vehicle you can build. DP