bumping up the rear to more freeway gears will give you increased top end with your 58 Apache trans (assuming it is the SM420). You need to do the math with the new gear ratio and tire diameter and figure out your rpm at cruising speed. You also need to figure out how you are going to use your truck. Your T5 may be a bit faster shifting since the internals are a bit smaller and lighter but, the SM420 is a workhorse, used in 1/2 ton up through 2-ton trucks. The T5 will probably have more evenly spaced gears, (SM420 has a big jump between 3rd and 4th).
My vote is the SM420 but, since that is what I run, I may be a bit biased!
