You have to go with your gut. I have a 100% stock 54 3100. 235 with 3 speed. I am comfortable running 50-55 for extended periods and have gone to 65 but anything over 55 make me uncomfortable. It just "feels" like I am running the engine too hard to maintain for any length of time. The other factor some have eluded to is handling. These trucks were not designed for that type of speed. The brakes, steering and suspension don't perform like a modern vehicle. You really have to be on your toes at higher speeds.
