The original question is missing one key piece of information - tire size. The diameter of the tire is going to make a huge difference in potential cruising speed. Once we know the tire size we can give an accurate safe cruising speed for that drive train based on facts.
We cannot, however, speak for the condition of the truck. As long as everything from the front leaf spring shackles to the rear leaf spring shackles is in great shape and properly maintained there is no reason one of these trucks could not be safely run at 70 mph. The drum brakes work amazingly well when properly maintained and if straight axles were so dangerous they wouldn't still be putting them under school busses.
My '53 GMC has served as my daily driver for the last 10 years. I am well aware of its limitations and its capabilities. I am always surprised when someone suggests that these trucks should never be driven faster than 50 mph. Even with the wore out old '49 216 ill fitted under the hood, I don't hesitate to run 65 - 75 mph for hours. Of course, I am geared better than stock and run an anti - sway bar.
