It sometimes amazes me what they put those poor little 318s into!
Grain trucks were geared pretty low and just lumbered along. Consider, also, that a lot of grain country is relatively flat.
We had an old Ford truck with a 292, 4 speed and a 2 speed rear axle. I was pretty adept at getting a run at the 1 hill I had to climb on the way into town and downshifting without losing speed.