You need to determine what gear ratio you have in that diff, I would not even entertain driving major highways with 4.10 to 4.78 gears, a whole lotta a revvin.
3400 rpm, is damn near redlined, 3000 would be okay, but closer to 2500 rpm for long highway drives is the ticket. Wait till you have a super hot day, and your running at 3000 rpm for a while, lots of heat.
You can swap in a different pumpkin( 3rd member), from another Mopar prior to 1954s, using a 10 spline axle. You could get a 3.54 from a 46-48 Chrysler, or a 3.73 from lots of Dodges and Plyms, relatively easy swap. Or get an 80s Mopar 8 1/4 diff, from 3.23 to 3.55, then you can cruise easier at highway speeds. I have P205 75 15 tires, and still feel comfortable at 60 mph, engine handles it easy, but these old trucks ride like old trucks. New shocks, king pins, tie rod ends, tight steering, springs in good shape, and good tires, are a must at speeds over 50 mph.
PS mark your drive shaft and rear wheel, roll truck until rear wheel makes 1 revolution,and count how many times the mark on drive shaft spins= diff ratio