The main reason for having a vacuum advance is for fuel economy. At light-throttle cruise speed with the manifold vacuum high, the vacuum advance cranks in about 8-10 degrees more timing advance. It goes away as soon as a load gets applied to the engine again. Since the really big rigs were seldom if ever run without a heavy load, there would be no reason for the extra advance. If you're not planning to work the truck like it was intended, swapping a dizzy from a lighter-duty truck shouldn't be a problem, but I doubt if there will be a noticeable improvement in gas mileage with the gear ratio and gross vehicle weight you're dealing with.
Jerry
