The nice thing about the '55-'62 rear is that you won't need to change the entire differential housing, only the 3rd member, which is the part that the gears actually ride in, would need to be changed. As far as the cruising rpm, an overdrive trans would be the best bet, as it would give you the better gearing for city driving and towing, but then also have the overdrive gearing for highway driving and feul economy, as well as reduced engine rpm.
