My old '63 C10 was comfortable up to around 80 mph, ok up to about 100 mph, but got scary after that. At the time, the truck was only 19 years old and had a 327/TH400/3.73:1.
My current '62 C10 is comfortable up to 65-70 mph, but can do short bursts of 75-80 mph. I have not tried to drive it above that yet. It has a 327/SM420/3.90:1.
Both trucks had stock suspension, brakes, and shocks, but modern radial tires.
