Are you sure you have a 4:10? Hope you counted the teeth and checked it out, should be a very noticeable improvement in speed in all gears. All tranys are one to one in high gear 3 or 4 speed same result in top gear.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed