While the SM420 is a direct swap, it will not give you what you want, other than being a floor shift transmission. 4th gear is still the same 1:1 ratio as the standard 3 speed, and 1rst gear is a very deep 7.05:1. The spacing of the rest of the gears isn't very good either. A better choice would be the T5 5 speed, which will have an overdrive gear, as would the MY6 4 speed found in early '80s GM trucks (Dodge also used this trans, but I don't know if the bolt pattern is the same).
