Help choosing new rear end gears

[Terence]

Hi,
I have a 67 C10 with a turbo400 transmission(3 speed) and I want to change the rear end gears so it will be a better highway cruiser and easier on gas. Right now, I max out at about 80kms (50mph) before it sounds like the rpms are too high on the highway. I have no idea where to start in choosing new gears for the rear end, and would appreciate any advice/help on what I need to be looking for.

Thanks,
Terry