I know this thread is a few months old but my friends 55 1st series came with the same 235 / 4 spd and 4.10 rear. His engine would also be pretty wound out at 50-55 MPH. He had the rear gears replaced with 3.38 gears (or close to that) and now he can keep up with the rest of us on car cruise days.
How do you think I feel with a 235 / 4 spd. and 5.38 gears in my 4400. My top speed is 48 MPH. 50 with a good tail wind. Gonna be changing that in the near future.

George