How hard/expensive is it to switch out gearing on an A-body?

Hey everyone, I've finally found a 4-speed Barracuda in my price range. However, it's a 340 with 3.91 gears. I don't spend much time on the freeway but I do like long drives on smaller roads. I'd like to be able to go 55-60 comfortably. How hard/costly would it be to switch to 3.55 or 3.23?