1972 Chevrolet Chevelle gears? I found out which ones...?

Ok so i have posted a few questions on this same topic. Im not sure what tranny my 72 chevelle has. its got a 383 stroker under the hood. When I cruise down the highway, I am going about 65 mph. I'm revving at about 3100 rpms which is a little high. Anyway, my dad and I asked his friend who has been doing transmissions and gears for about 15 years. The friend told us 3.42 gears are the way to go. When we put these in, will I be alright for traveling at 65 mph at about 2000 rpms? And will I still have all that raw power when I smash the gas and want to hot rod a little? Im not sure on anything else, all I know is I have a beefed up auto tranny, 9 inch wide tires on the back, and thats about it.