If it liked 20 initial, you are fighting an uphill battle.
Detonation is caused because it has too much advance at certain points in the curve when under load. Less mechanical advance would be the first thing to adjust. 38 is apparently too much for your deal, so back it down to the 34-36 range and see if it runs without detonation. Pulling initial out is NOT the answer.
Too quick on feeding in mech timing can be a cause, not usually. That is controlled with varying spring rates on advance weights (faster or slower feed in rates.)
It WILL run fatter with less initial timing, both at idle and when you crack the throttle.
The break in thing is BS, it's not the 60's with machining tolerances. How many miles are on an engine destined for a dyno... they aren't babied when obtaining peak performance numbers.
