I see no problem with kingpin front ends and speed. Every big rig I know of has a kingpin front end and most can go plenty fast.

The only concern is that they are not worn out.
As for brakes, the old brakes on our trucks should be fine at moderate speeds if they are in good condition and you are unloaded. As you approach a full load I would try to stay under the max speed the truck would go originally, about 45 or so for a 2 ton. Experience will tell if they are good enough for speeds faster and with more weight.
If you want to go fast all the time I would at least upgrade to a dual circuit brake system / master cylinder.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-