The SM420 was an option on the lighter trucks, however, the parking brake would only work on the 1 ton. The 1/2 and 3/4 tons use a torque tube/ partial torque tube driveshaft system, respectivley. And even at that, I'm not so sure that the yoke for the 1 1/2 ton trans would work with the 1 ton driveshaft, plus, the 4 speed 1 tons actually have that style park brake, only smaller. If you've converted to open driveline, and have a driveshaft made with the right U-joints, then yes, you could use it.
