Hey, this is in regards to my 'Cuda 440-6, 727, 3.55, stock-ish build. I'm just starting to drive it, it's a ground-up resto, so it's all new. Engine was thoroughly tuned on the dyno.

Car seems to run great in almost all situations. When I got on it 100% today though, I got what seems to be a LARGE backfire in the driver's header collector. Only at WOT. I got off of it right away, the first time I thought it was some sort of breakage. Felt like a hammer hitting the floorboard. I didn't see anything wrong though, and the problem seems to happen pretty consistently, but only at WOT. Any suggestions where to start with this one? Thanks.

There was a well-written thread on here about setting-up and checking the function of a vacumm outboard sixpack on here a few days ago. It pops up from time to time because it's very informative and hits all the bases with regard to tuning and function. Do a search for it and check the basics ie fuel flow, linkage adjustment, timing etc.