I am not sure what is so hard about driving an SM420. My wife drove the one in our 59 Suburban for 20 years with no issues. I had a 79 Blazer rear in it with 3.40 gears, (also changed out front end, added power disc brakes , and different leaf springs) and I could cruise at 90mph all day long with no issues. True, an OD may be nice to drop rpm at that speed and maybe pick up some gas mileage but, again, it depends on how you are going to use the truck!
Dongray, I would see if there is someone close to you that has a truck with an SM420 in it that will let you drive it. You may find that it is not a big deal and you like the granny gear!
