welcome to the site! If your truck is a 6400 series, it should have a 2 speed rear end, does it? 6400 series is a 2-ton. If you have 22.5 wheels on it (tubeless tires) you should be able to get 9R22.5 tires (at least on the rear, might need a bit shorter on the front for turning clearance).
My 58 2-ton will go 70mph @ 4200 RPM. 3600RPM is 60 mph just so you get an idea. (I have 9R-22.5 tires all the way around).
