Might want to double check your Vac Adv. to make sure it's advancing your timing as needed? Might need to reset the initial timing, and check the dwell, and make sure your Vac Adv. is actually advancing...

What heads do you have on it? Maybe #16's? With Higher comp. heads, you'll have to really dial in the timing sometimes to avoid detonation...

Hope you get her all dialed in and set!

Brett - 67 Drop Top 455!