What size wheels and tires does your truck have? I ran a 59 1 ton (3800) for several years, with a 4.56 rear end gear and 18" wheels. It would run 50-55 MPH without doing any engine damage, although it did sound a little noisy at those speeds. The 216 and early 235 engines, one of which your truck probably has if it's original, were designed to run considerably more RPM than people who drive later model trucks are accustomed to hearing. Just for grins, try removing the fan blade and make a highway run. I think you'll be able to get a better understanding of where the noise is coming from.
Jerry
