Faster gears will help.

Also adjust and tune your engine, 35 mph is very slow even for the stock setup.
Adjust the valves.
Set timing, for a likely good setting try 10 degrees advanced at an idle with vacuum advance disconnected. (Reconnect before driving)

You should easily and comfortably achieve 55 mph stock.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-