Are you doing "real truck" stuff with the C-60? If so, you'll need to disable the vacuum advance on the HEI, and rework the centrifugal advance to provide a total advance of around 34-36 degrees at 2500-3K RPM. I'd suggest setting the initial advance at around 10 degrees, and modify the movement of the centrifugal advance weights to provide another 24-26 crankshaft degrees. (12-13 distributor degrees) The rate of advance, (not the total number) can be controlled by tweaking springs. The only purpose for vacuum advance is to improve fuel economy, something that's just about impossible on a C-60!
Jerry

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