The primary (really the ONLY) purpose for vacuum advance is to nurse a little better gas mileage out of an engine at light throttle cruising speed. An engine that's not lugged down or trying to accelerate can tolerate about 10 degrees more spark advance than one that's pulling hard. Big trucks and good gas mileage are about as compatible as oil and water. There was no need for that extra advance, because it was seldom, if ever going to happen- - - -there is low to no manifold vacuum at full throttle on any engine, especially one that's pulling hard most of the time. The centrifugal advance just has to be set up ro deliver maximum torque at the desired RPM, which requires a close coordination of compression, camshaft profile, and ignition timing. Aircraft piston engines also don't have vacuum advance, and sometimes even the basic spark timing is manually controlled. Ditto for Model T and Model A Ford engines, and 1920's and early 30's vintage Chevy engines.
Doing race engine stuff, or heavy truck stuff on a street engine in a light truck or a weekend cruiser is beyond foolish.
Jerry
