1977 Chevy Silverado Timing mark

Is the rubber in the harmonic balancer coming loose? Are you using a fluid damper balancer? Are you able to get it close to 7 degrees before TDC?

Could be coming from the vacuum advance unit/worn distributor cam lobe Hence: Dwell angle will change ignition timing-Ignition timing will not change dwell angle.

Check for wear on the distributor gear, I prefer brass gears.