What year is it, and do you have the 3200 RPM governor spring?
The NV4500 swap is a little more involved than bolting the other transmission up. It takes the flywheel housing and starter from the 94-98 trucks as well.
The 6 speeds don't have any trouble with 5th and 6th gears coming apart.
If you have any questions about an old square-body truck the guys at the DTRF can answer it.