The SM-420 is a sycronized transmission and is far better than your non-sycronized trans. They both have 1:1 high gear so the travel speed will be the same.
As far as the drive shaft, that's the easiest part of the project...don't let that be a deciding factor. Grab the drive shaft out of the K10 so you have the correct trans U-joint flanged yoke. Then contact you local drive shaft shop and have them tell you how they want you to measure for the new shaft. Between the K10 shaft and the one in the truck now you should have all of the expensive parts needed.
Mike B
