The internals from the o/d trans will not swap with the standard trans. The overdrive variant uses a different casting for the housing, as well as a different mainshaft. You can swap some parts, such as the countershaft, 1rst/reverse slider, bearings in the main case, and seals.
If you are wanting to just install a non-o/d trans, as long as the length and spline counts are the same, it will be like a standard trans install. You will need to remove the o/d control hardware. How bad are the gears in the o/d? The gears for the main trans are available through several sources, as the are the same as non-o/d. The aren't very hard to work on either.
