Rebuilt 318 install problem

I watched that video about 20 times, and have never seen something like that without accompanying flames. It sounds like a pressure spike hitting the underside of the throttle plate.
So I'm guessing crossfire/backfire. But it sounds like just one shot ,every second revolution. And the engine idle just doesn't sound right,either.
It could initiated inside the cap;one every second revolution. But it's really hard for me to imagine one per 2 revolutions that way.
That just leaves three things; two crossed wires, a poorly synced reluctor, or if the advance weight springs are so weak that she begins advancing almost as soon as the revs begin to climb and then for some unknown to me reason, she crossfires to the next tower which is 90 degrees behind and still on compression.
Since OP is insistent that the firing order is correct, that kindof just leaves an induction misfire;and that usually only happens between wires 5 and 7, if they are routed parallel and too close together. But I struggle to figure out why or how it could be load/rpm related as shown in the video. It's almost like just one header pipe is plugged backing the pressure up into the intake; but there are no headers.............
I think what I would do is to isolate which cylinder is doing it, and attack it from there.

But that still leaves the odd sounding idle. I guess you could hook up a vacuum gauge and see what is tells; if very low,maybe the intake is full of EGR
She's a stumper for sure, and I'm eager to learn what this ends up being.