Before you switch to a SM420, I think you should drive one. You say that having the low first gear doesn't interest you. You will find that reverse is just as slow. Second is synchronized, but it is also slower than a SM318. Unless you plan on pulling trailers, you probably would find a SM420 a nuisance. Again, I would urge you to drive one. If you like it, then I could consider making the change. If the drive shaft has a center bearing, it probably came from a 3/4 ton. Putting in a cross member for a center bearing is no big deal. Or if you can get away with a single drive shaft, make one out of the drive shaft that goes from the center bearing to the rear end, you should be able to have a longer tube put in it for less than $75.00 I have 1700 and 1800 series shafts made for about $60. You probably have a 1300 series which should cost a little less. But $75 should be the max even if you are not a regular customer.
