Someone can correct me if I am wrong but the speedometer gears come as a set. One is pressed on to the u joint yoke and the other screws into the transmission case. You may have to change both with the SM420 ( I know you do for the predecessor 4 speed crash box tranny). That would be a kind of big job, but very doable. You would eventually find a set on Ebay.
It would be much easier to have an adapter made that just takes a minute to install. You can try some of the speedometer shops that advertise in Hemmings.
Kent
