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.