While cruising at say 3300 rpm's, which is about 60 MPH with the 3.73 gears, if I moderately push the gas pedal the car will start to accelerate but then will suddenly backfire(one big big pop) through the carb.

What is the cause of this? Is that what's called a "lean" backfire? The carb is an 870 Holley Avenger with stock out of the box jetting ,which is I think is78/86.It has power valve of 10.5

Solutions/suggestions/diognosis please!!!

Engne is BBC LS6 shortblock 454 with GMPP large valve alum. heads.Comp Cams XE274 H .Performer RPM intake and MSD 6AL.

How does it run, say from a stop? Does it backfire when you go thru that rpm range or from the launch? Timing? larger squirters maybe?