Mile High Perf Dyno Tune 340

Disappointing runs at Brandimere. Best was 15.4 @ 93. I suspect that sometime in the past the vibration dampener slipped as one last year I found the initial timing 20° retarded. I changed the distributor drive gear to compensate, but that was not the root cause of the problem, and probably masked the issue. I need a TDC tool to validate the timing marks on the dampener and do a root cause analysis to be sure all is well with the engine.
One clue is that I show 15° initial and 40° total with no detonation. The small blocks are supposed to run best with 35° total, so something is amiss.

Any thoughts out there?

Thanks,
Bob