I routinely cruise my 56 3100 at 70mph with straight axle and drum brakes. That's Sierra mountains, flat valley lands as well as bad California traffic. If your truck is well maintained and in good condition, no problem. Never had brake fad and have always had the truck under control. I have had this truck since the 70's -- no crashes. Do not tailgate, drink, eat or yak on the cell phone while driving. Trucks take longer to stop than a Honda Civic, so think defensively.
I used to have 3.90 gears with 650x16 tires. I now have 235x15 tires and 3.38 rear gears, much nicer at 70mph.
