Loud pop and sputter on WOT? 68 318 Barracuda

Traxfish said

Yes, it's a BBD.

I did try something new with the initial timing. I used to stick it at 10 deg, then backed it down to 8, but this time I advanced it until the engine started running rough, then backed it down a couple degrees. Ended up around 14 deg btdc. I have also been considering this is the culprit and I need to take it back down to 10. Engine seemed happy there otherwise.

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If she got unhappy at 14* advance, then something is most definitely wrong.
I'm assuming you set the timing with the V-can defeated and/or plumbed to the inactive at idle, sparkport.
If you say yes, then she should have been happier with up to 30*, idling in Neutral.
I highly recommend to verify that your balancer has not slipped and that the Piston TDC coincides with the balancer TDC. This is done with a piston stop.

If they are synchronized, then I would check the cam-timing.