I must say that from the video it seems that the knock is on every crankshaft revolution. Whether it is on every crankshaft rev or every other is important.
If half speed, that means valvetrain, so perhaps a collapsed lifter.
If on every rotation, that implies crankshaft.
Now, take another video and get the revs down as low as you cen get the motor and with the fan and pulleys in the shot. We don't need to see the air cleaner nor hear you rev the motor. The slower, the better and we want to judge the sound to the movement, hence seeing the pulleys/fan.
If it is something like a rod bearing, using Seafoam or MMO is the worst thing you can do. These are thin oils. The motor wants thick as you cannot replace missing metal and thinning the oil even more makes matters worse.
It is possible that you wiped a rod bearing as a result of the oil not flowing on that cold start you mention. On any frigid start, let the motor run at no load and at idle speed until you get some warmth into the lower end. The motor can run all day at no load/idle speed with insufficient lubrication but as soon as you intoruce load (by driving) or increase the revs it can be bye-bye time.
BTW if that knock is on every crankshaft turn, the swapping distributors was irrelevant.
I did not read the entire thread but assume that the basic troubleshooting steps have been outlined.
You do not need a stethoscope. Just get a long screwdriver, dowel, welding rod or whatever. Press one end on what you want to listen to and the other end against the bone behind your ear. It is all you need to hear any engine noise and you will amazed at how well it works.
Assuming you have a bearing knock, I would run 20W-50 with plenty of Lucas oil additive. You want the thick oil to create a cushion so that the knock does not get worse.