Welcome aboard!
The (RPO) MY6 from an 82-86, sounds like what you are looking for. The MY6 is a New Process A833 designed for Chrysler and used behind everything from the slant 6 to the Hemi, and in the 3/4 Dodges. Fourth gear is an overdrive. The MY6 will bolt right in. The bad news is you will end up with a floor shift just in front of the seat, and the drive shaft will have to be shorten about 9". Another option would be the T-5, fifth gear overdrive. The t-5 wouldn't be good for hauling a serious load.
I have the MY6 in my 63 C20, and like it. The gears are comparable to 2-4 of the M420 which you have.
Larry
