What gears for my 67' f100 rearend?

My old ford Is fun to drive but when im on the highway im running a bit to high in the rpm's. I want to be able to roast of the tires still but i want to be able to drive a little nicer at faster speeds. I know alot has to do with the old automatic 3 speed tranny in it but i cant afford the rearend and tranny right now. + the tranny still shifts good. So what do you recomend?