Hi, I experienced something similar in my setup. We noticed first at the dyno session and noticed some bumps in the graph and inside the car it felt like hesitations, we did not hear misfires or backfires at that time. played with the timing starting from 34 down to 30 or so, ended up in the middle ground of 32 and left the full advance until 4k RPM.
Originally the can ran kind of lean (13-13.5) with the 426 matching map, later I moved to a much larger engine map and now AFR is right where the target wants it to be (12-12.5) so it improved considerably, this allowed me to also set the full advance to 2.6k RPM, which is a lot more responsive, in parallel I pulled back final timing to 30 degrees.
When driven hard I noticed some times that hesitation at RPMs above 4k and sometimes heard misfires very seldom. Given the fact that the car has a 426 stroker and 2.73 gears I don't rev it up past 4kRPM that frequently (360HP @ 400lb-ft at the wheels at 4500 feet above sea level, using pump gas).
But your post brought this small issue immediately. When setup we set it to the 12 degrees fixed, then when outside we timed it with the light gun and it matched the edelbrock computer readings, but I don't remember how high we went in measuring it still matched. Would a good test be to check the light gun vs the Edelbrock ECU readings with the engine running up to 6k RPM?