Chevy used the same basic carrier design in their 3/4 ton (and 1 ton, iinm) pumpkins from 1946 - 1972. The actual gear parts are different but all carriers work in all pumpkins.
As it was stated before, the 68-72 style carrier came in a couple of different ratios: 4.57, 4.10, and 3.90. 4.57's are by far the most common, followed by 4.10's. I'll repeat GMCPanel's wishes of luck on finding a stock 3.90.
The good news is, you can purchase 4.10 and 3.90 gear sets from
Jim Carter that will fit the 68-72 carrier, if you can't find one with stock 4.10 gears.
The only thing is, they're pricey.
Good luck in your search!
