I replaced the sm420 in my 63 C20 with a GM RPO MY6 GM equivalent of New Proecess A-833. Chrysler used it in everything up to the D200 truck and behind everything from the slant six to the Hemi. Forth gear is an overdrive (.76). I've got a rebuilt 292 still pretty much stock, 4.10 Detroit Locker, first gear is great, second gear is almost freeway speed. The drive shaft had to be shortened, but the shifter ended up right in front of the seat.

Larry

I don't own a vehicle that isn't old enough to drink.