HEI module going spastic?

I watched the video, but didn't see the HEI module. Not sure where "under distributor" means. Someone made a plate to mount HEI under a slant six distributor. Do you have similar on your small block? I like your diagnostic. I do the same when checking spark. You shouldn't have missed sparks at idle since it is easy for a spark to jump the gap at low cylinder pressure. It is under WOT at low rpm where one gets misses from an excessive gap.

Did you tightly twist the wires from the distributor pickup to the HEI module? Not doing so can give cross-talk which causes spontaneous firing from positive feedback. I experienced such when testing with my HEI module using temporary jumper wires. Perhaps your #3 plug wire runs closer to the pickup wires? Why this matters - an open loop is an "antenna" which picks up high-frequency magnetic signals like that generated by the current in a spark wire. By twisting the wire, you form multiple loops in different directions, which cancels out any pickup. Another thing is to verify the HEI module is well grounded.