The '63-'76 C 10s used the Truck version 12 bolt. Aftermarket gears are readily available from many vendors, Randy's Ring and Pinion seems to be one of the better ones on price. Just about any ratio you could want is available, from the 2.90 all the way up to a 5.88 (not suggested, 1/8 mile drag race gear). Yoyu may have to change the carrier depending on what gear you want to run. Side note, '63 and '64 truck 12 bolts use 17 spline side gears, those are not available aftermarket. If you need to change them, or want a limited slip, you will have to change the axle shafts to the later 30 spline.
