I agree with all said about top speed. From 40, 45, 50 MPH, is about it for the normal working truck.
I have a 1950 6400, and I enjoy the cruise to do a bit of work with her bed and hoist, and dumping the load, or gravel, is all for the fun and use of it. Not for burning up the road ! eek
Ha ! Just to look at them is what its about, especially when recalling learning to drive on them in the 1950's !!

MJD