[Mike C]

Having messed with multi-carb setups on 240z and motorcycles, usually a backfire out the carb is a lean condition. Needs more accelerator pump shot, faster pump cam timing or just to be enriched. A 600 cfm carb is probably set up WAY lean for a cammed 383. See if moving the stock pump cam to a the second position makes a difference (I'm assuming a Holley carb but if not check how to adjust accelerator pump voluem/timing on the Edelbrock.) Go up several sizes in the shooter and see if that doesn't improve things.

If it runs fine at idle and at cruise speed, it seems to be a transition thing which is either idle circuit too lean or pump shot too small etc. Increase your idle mixture richness as well. Sometimes that improves the transition between part throttle and full throttle and will cover the "hole" or backfire that's created when you stomp on it.

On Webers of Del'Ortos, the "idle" circuit controls mixture until around 3000 rpm when the mains come in. So it's imperative that you get that right or you have all sorts of driveability issues.