[66Submarine]

Neat truck. I drive my '65 C-30 as a daily driver. You have pretty short tires on your truck, and may not have 4.10 gears. (4.57 or 5.14 is what they came with new) You very probably have an SM420 Muncie four speed--if it has a very low first gear and you shift 2-3-4 that's what it is. M21 will only make the gears closer together. Disk brakes can be taken from a 71-87 3/4-1 ton, or burb up to '91.