This isn't a cheap option, but you can go with a Gear Vendors O.D. It bolts on the the sm420 via an adapter that they provide. I had to have my driveline shortened, my torsion bar crossmember slightly modified(I don't think you will need to do this with your truck) and a custom rear mount fabricated to hold the extra weight. The adapter provides a nice mounting spot to fit a stock mount. With this O.D., you still have your granny gear, and you can overdrive 2nd, 3rd, and 4th, giving you 7 forward gears. IMO, this gives you most versatility. By the way the O.D. is advertised to be able to handle 1500 hp. Your strong SM420 will break before the O.D. will.
