Can I assume the sm420 was used on the half ton, 3/4 and 1 ton models?
Yup! The shifter will be about 4 1/2 inches from the front of the transmission. In fact, the transmission you likely have is the sm420, but with the torque tube rear. You might be able to swap just the rear yoke of the transmission with that of an open drive sm420. It is probably easier to swap the whole transmission and then sell your closed driveline sm420.
