Here's the problem: misfire and backfire through the exhaust above 3,500 rpm, in any gear, "MOST" of them time (not always) typically 1x out of 20 3,500+ rpm run ups will it NOT backfire/miss.

Car's a 340 6 Pack automatic with all 3 Holley carbs running well. Car was on the dyno and is not running lean (before this all started happening).

Here's what I've changed or replaced:
Cap, rotor, plugs
Have checked the wires with a spray bottle
New Orange ECU box (have a Chrysler electronic ignition conversion)
Ballast resistor
tried a friend's known working distributor
Some even suggested the Sun tach be removed from the equation. Done.

Here's another thread before I heard the backfire and changed the distributor: https://board.moparts.org/ubbthreads/show...&PHPSESSID=

Thoughts?