As long as the brakes can stop the tires you have enough.
That's not strictly true, since the sliding friction of the tire is less than the rolling friction. Metal bars shoved through spoked wheels would also stop the tires and wheels, but would likely result in a longer stopping distance compared to more conventional brakes.
One of the reasons I posed the question in the first place was because I too had been told a theory similar to that mentioned by Joe H. in the second post: Disk brakes apply the braking in a more linear manner than drum brakes, which in practice allows them to more closely approach the point of lockup before reaching it.
But this effect is not likely significant, if it even exists (at least for Huck brakes, which are not self-actuating). Clearly Denny's truck displays exemplary stopping performance, equivalent to a modern 3/4 ton truck. This tells me that the replacement of the Huck brakes would not be worth the money or the damaging effect to the truck's appearance.
Thanks for all the responses.
