Mr. tyler h you hit on much of the fuel mileage issue on your own. These old trucks have low rear gears. Taller gears and/or OD transmissions help a great deal.
That '68 that had the 292 and SM465 probably had 4.56 gears. Your '50 has that or lower.
I changed the 4.56 to 3.54 gears in my 4700# 3/4 ton and measured by the seat of my pants, it has more pulling power.
Mileage is about 16 with a 80% / 20% city / freeway. I'm not very diligent about keeping track of it.
