Welcome to the Stovebolt!
Task Force trucks were available with a 19.5 inch wheel that will fit your truck. You might look for a set of those.
The Eaton rear under your truck was available under 3/4- and 1-ton trucks until about 1972. If original your gears are either 4.57 or possibly 5.17 (an option) to 1. Later trucks had a 4.11 gear set, and rarely a 3.70 or about that. I haven't heard anyone say they found the 3.something gears recently, so the 4.11 is probably your best bet.
Alternatives are switching rears.
