Most likely the "Not for highway use" is there to limit any liability by the manufacturer.

'63 Dodge 330

11.19 @ 121 mph
Pump gas, n/a, through the mufflers on street tires with 3.54's. 3,600 lbs.

9.92 @ 135mph with a 350 shot of nitrous and 93 octane pump. 1.43 60 ft. 3,750 lbs.