Like I said, buy a drop-out center section with the gears you want already in it. Then, you've still got the original setup to put back in without hassle.
Unless you have some really small diameter tires, you should show 60 or more at 3500 RPM with 4.11's. My '54 Bel Aire with a 283 and 4.11 posi and 15" rubber did.