[1969guy]

Also depending on your driving, I would change your vacuum advance over to manifold vacuum and not ported vacuum if that's how it's setup.

The difference is that when your truck is in neutral or park and at idle, if you pull a rubber line or cap off a vacuum port and get instant, fairly strong vacuum, then that's manifold vacuum. Ported vacuum only increases if the throttle plates are opened up (acceleration).

Now putting the rubber line on a manifold vacuum port will instantly shoot your timing up how ever many degrees it is rated for, stock gm's were 20. So you need to set your base timing somewhere in the neighborhood of 3 to 6 degrees before the rubber line is attached.

Now the reason for manifold vacuum advance in a high compression engine is that if you step on it (fast to moderate acceleration) the timing does not jump higher, as if ported vacuum were used, than your base and centrifugal timing, which increases the chance for ping if the octane fuel is not correct or the quality of fuel is just not there(today's pump gas). When you step on it, the manifold vacuum advance setup will allow the timing to back off some and reduce the chance of pinging.