Low gear or trans or both?

[LilD]

The 70 C20 I just bought revs to the moon at highway speeds. It was a 6 cylinder to begin with but now has a V8. The trans is the original 4speed with the "granny" gear. I was told that the 6 cylinder trucks had a lower gear ratio in the rear end to make up for some low end grunt. So is it the gear in the rear end or the trans that has too low of a gear or a combination? It sucks not being able to enjoy a cruise down the highway and watching the speedo needle and gas gauge needle meet.