You might want to consider an New Process 440/RPO MY6, aka A833. It is a 4spd and 4th is an over drive. It was originally designed for Chrysler, used in everything from the Duster to the 3/4 ton trucks, and behind everything from the slant six to the Hemi. Here is a link that shows where they were used;

http://www.drivetrain.com/parts_catalog/...erhaul_kit.html

All the trans under Chevrolet and GMC would be a direct bolt in. You may have to shorten your drive shaft. You would also end up with a floor shift.

Larry

Ignorance can be fixed Stupidity is forever