I actually ran into the same situation on my 52 3600 and found that 3.90:1 is about the tallest [highest]gear ratio you can get for that carrier assembly. We just swapped out the R&P gear and a bearing and it worked fine. I can't swear to it, I am sure someone else here at Stovebolt can verify but I beleive the 3/4 ton carrier assembly is the same up until 1972. Any gear ratio taller that 4.47 or 4.11 will help especially if you do any interstate driving.
JimBo
