REAR END upgrade question.

[junglefire]

I have a '66 C10 with a 350 crate motor..
when i get on the highway and get up to 55-60mph, i feel like the engine is revving high and pretty much can't go much faster. i'm sure my gas mileage suffers here too. i don't go on the highway often, but when i do..ugh.

is there a rear end i can swap out. what gear ratio should i be looking for?

just wanna get up to 60mph with out over revving.. i have a 3 on the tree tranny.