tranny swap

This would be more exact if I knew specifically which trans and engine you have, and what you are swapping in, but generally, you'll need to know the following:

The trans will physically fit in the truck. However, you'll want to check pilot bushing sizes, and clutch spline count for the input shaft. The bellhousing from a later model (assuming you are using NV3500, NV4500) Dodge five speed should bolt right up. Be sure that the clutch disc and pressure plate is able to fit the flywheel you have. If not, a flywheel from a late model truck should bolt up, but you'll want to check this carefully. Then, you'll need to figure out if the new trans will allow you to use the clutch linkage in the truck, or if it will need hydraulic lines set up.

Beyond that, the driveshaft would be a concern. You'll want to check the type of ouput, whether its spline or a flange. If its a splined yoke, you'll want to be sure its the same as what you have. If not, you'll need the yoke from the "new" trans and a ujoint that fits the present driveshaft and the "new" yoke. Then, you'll need to check driveshaft length.

This swap will take a bit of time and engineering, so count on the truck being down for a few days, unless you get all the specifics ahead of time, and have the means to assemble all the parts. If you can do that, it'll take about a day, maybe two.