These trucks were designed for about 50 mph. Above that speed and you don't have enough brakes or steering stability to operate with a reasonable level of comfort.
One thing to remember, safety is relative. If you want to be absolutely safe you live in a bubble. You can do things to improve your margin of safety, such as being sure your brakes are up to snuff, the steering is tight, shocks are in good working order, tires are in good shape and properly inflated. Even with all that, I still would have concerns driving a stock truck over 60 mph.
Another reason for my 60 top speed is the engine. The stock 216 was not designed to operate at high speeds. The rod bearings are poured babbit, which is soft and flexible. And it is thick enough to deform. High operating speeds cause this deformation, which results in rod stretch and premature wear. After a while, you end up with an engine locked up due to rod binding. It is concensus here that 2,700 rpm is a good upper limit for the 216, which is between 55 and 60 mph.
