In stock configuration with all components working as they should, there is no reason you can't drive safely at highway speeds.
I drive a 1950 3100 in stock configuration (including Huck brakes).
New radial tires and wheels, and shock absorbers are the only new parts on the front and rear suspension. The tires are large diameter, close to stock diameter on 16" rims.
216, 3spd, 4.11:1 rear end.
I drive the truck weekly on the freeway at 60mph with no craziness or scariness. I can let go of the wheel and it tracks straight. A newer truck with a bigger engine like yours ought to drive and handle just fine at 60-70mph.
How much of a weight difference is there between your new, souped up drive train and the original drive train?
