Holley 750 backfire.

4.10s are good, Is it just a standard MP dist.? If so, you could tear it apart & weld up the slots & install the light springs, but its hit & miss on the slots, you may need to tear it down a few times & add more weld or file the slots till you get it right.

How off is the V pan with the port allignment on the heads? You don't want to use any silicone on the ports, while its idling, spray under the VP towards the ports & see if theres any change. Where is the int. timing at now at idle??

Are you getting a steady stream of fuel from the squirters throughout the throttle?