My vote is for an RPO MY6 (GM 82-86) aka NP A833. Designed for/by Chrysler, run behind everything from slant 6 to Hemi, and everything from Duster to 3/4 ton truck. The MY6 is a side shift 4 speed, it has about the same gearing as the sm420 2nd, 3rd, 4th, for 1st, 2nd, 3rd (1:1) except 4th is an overdrive. You'll need to have a bell housing with or open up to 5.125 for the bearing retainer, and the drive shaft made to fit.
Larry
