Timing/ spark knock relationship

When I recently rechecked the timing after adjusting the rockers and pushrods on my PU, I noticed the timing mark was extremely advanced. If the scale on the timing tab read to 18 degrees, that's about where it was flashing when No. 1 TDC fired. It should be at 8 degress BTDC. I suspect the vulcanized center and outer ring have seperated and the timing mark is not calibrated. When adjusting the distributor for smoothest idle, the road test results in a very obvious spark knock. When readjusting the distributor, I can eliminate the spark knock, but the idle and initial acceleration to about 10-15 MPH are very rough. Is there a happy medium here? Haven't been able to find it. I followed service manual procedures for adjusting the rockers and pushrods, but fear I may done something wrong. A new harmonic balance will restore my correct timing mark, but there has to be a bigger issue here. BTW, no vacuum leaks, new plugs and wires and replaced bad component in distributor. Thanks for the input!

did you disable any timing advance mechanism first?

Yes, I disconnected and plugged the vacuum hose at the distributor, as directed in the service manual.

Is there also an EST connection? there were some goofy crossover models that used a vacuum and electrical advance

No, this is a 1980 GMC. Pretty basic and has been reliable. I should add the vibration is most noticable when the truck is idling in gear or accelerating to about 15 mph. While idling in park or neutral, it is smooth. Thanks!