BRAND NEW 305 H.O. AND NEW EDELBROCK 600 CFM ELECTRIC CHOKE WANTS TO BACKFIRE WHEN YOU MAT THE PEDAL HARD, THIS IS WHEN THE CAR IS IN PARK, YOU CAN BRING THE RPM'S UP TO SPEED SLOWLY AND NO PROBLEM, NOTHING HAS BEEN ADJUSTED ON THIS OUT OF THE BOX CARB EXCEPT THE IDLE SETTING WHICH I PUT AT 600 RPM

 

Just guessing, but it sounds like a timing problem. The carb may be too rich, as well.

 

try adjusting you fuel to air ratio. not to make you sound like an idiot but that is the 2 screws on the front of the carb. I think the left is the fuel and the right is the air or something like that. I don't think it is in the timing bu it might be. I am a rookie at this so don't just take my word. But that is what I would try. Does not cost anything but a little time.