what would be your diagnosis?????

what dampner are you running? if it has an elastomer in the center of it. mark it with a bright white or yellow across from both sides. see if its moving more than an 1/8" to one direction. they do wiggle some but it could have slipped at initial fire and you never knew it.

is it internal or external balanced? u say you had it balanced but is that with the dampner and flexplate?

id suggest swapping out that junk 6al box for a digital 6. common random misfire is the 6al's regular failure flag.
what is plug gap? you can eliminate plug gap by going dowj to .014 and rule out any ignition fire issues. but once again 6al box with the low rpm multi fire delight is also its freddy kruger for these type of problems.

did you check dizzy engagement for full gear contact? is it a tooth off?

those are the things i consider once ive ruled out carb, valve back lash, lower intake leaks, and dizzy damage.

you could also have had the cam bolt if center single bolt cam back out allowing chain deflection for a rough runner.

cam advance sprocket fall out or come lose if adjusting bolt style not incremental spacer designed.