I don't know if you ever said what kind of car your 3 speed trans is in, but most of the GM compacts (Chevy Nova, Olds Omega, Buick Skylark, and Pontiac Ventura) all used 3.08:1 rear end gears with six cylinder engines, except for the station wagons, which had 3.36:1 gears. I think one of the first things you should get is a tach, so you'll really know what your rpm at a given speed is, and the second thing is to get a copy of Leo Santucci's book and read it before you start buying parts.
GM pickups did use lower rear end ratios than cars did, so if you have a truck, it may have a 4.10:1 rear end. I have a '68 Mustang with a 3.50:1 rear end in it, and it turns 3500 rpm at 65 mph according to the tach. One of these days it'll get a 5-speed. Back to your car, though, one of the main things involved in building a hot rod, or just modifying what you've already got (which is pretty much what you're doing) is knowing what you're starting with and figuring out what you want to do with it and how to make everything work together so it's fun to drive after you get finished with it.
