What size truck are we discussing here? Lots of the bigger GMC engines did not have a vacuum advance, as its only function is to improve fuel economy at highway speeds, and gas mileage on a big rig can best be described as "slim to none". If your truck is bigger than maybe a 1 ton, there probably won't be any vacuum advance to worry about. Use a delay-flash timing light and set the maximum advance at 2500-3000 RPM at around 35-38 degrees. Let the initial advance fall wherever it may, as long as there's no difficulty starting. 30-somethuing years of doing dyno runs on all makes and models of engines has convinced me that unless there are very special circumstances like radical racing cams and/or high compression involved, full throttle spark advance in the mid to high 30's works on virtually all engines.
Jerry
