If it leans out and backfires when you step on it, how's the accelerator pump on that Holley? Did you bolt it on new? If not (or maybe even if so) does it have the throttle shaft cam that actuates the shot pump rocker? Is that rocker moving up and down, if you look in the carb throats, do you see a solid squirt of fuel when you work the throttle? If its an old carb, the pump diaphragm could be dried up and not providing enough fuel, just hanging up halfway activated all the time.
