35 - 40 mph are an economical cruising speed. They are not a highway truck: there were very few turnpikes in those days, and the interstate system was still a dream. 48 - 50 was a flat-out max, because if you could get to 55 it would hammer your engine apart fairly quickly.
That said, grab it if you can, but plan on a diff change. The truck will be much happier.
