gtx

OK, car guys. I need some help.
My '69 Coronet is 99% ready for the road, but having installed a big block, and sorted out electrical, fuel, steering, vacuum, and all the other stuff, there's still one problem.
When I put it into D and step on the gas, she dies.
There's a pop, like a backfire, which is simultaneous with the dying.
Car starts fine, idles fine, you can manually advance the throttle in P and she's fine.
She's drawing 14-15" vacuum, there's fuel -- I just rebuilt the carb, a Holley 750 vacuum secondary.
There's three carb vacuum ports, one for vac advance, the other two are plugged.
Timing is set to 7 1/2 BTC, per shop manual.
It's a bored-out Mopar 383, I don't know what the cam is, but there's an Edelbrock intake, headers and a 3" exhaust.

I'm taking suggestions.
At this point, since I live on a busy street and continually stalling in traffic is a problem, I'm going to jack it up and remove the rear wheels and test her in 'D'. Unless someone tells me that's insanely dangerous, which it might be...