My 71 C20 with 350/4 speed came with the 4.10 rear gear. What size tires are you using? I have tall tires on mine and it sails along nicely at high speeds and fairly low RPMs. In fact, it loves the big highways. I never tried to get it up to top speed but it certainly would go 100+
Maybe you have the 4.56 and using short tires too?
There is a way to tell ratio. Lilft tires off ground. If posi, just put a chalk mark on driveshaft and one on tire. Turn tire one revolution and count how many times the driveshaft turns.
If non posi, same procedure but you have to turn the tire twice.