Pull the plug wires off one at a timeand rev it so it makes the knock. When you get one that makes the knock go away or get decidedly better, there's your culprit cylinder. Sounds like a rod bearing gone to me.
Pulled #6, no change. As soon as I pulled #5, I heard the knock, softly, at idle. Engine rev: nothing. Checked the rest just to be sure, they're fine (for the moment)
It will be cheaper to change out the motor. I shall continue trolling the forums for one.
I'm not overly surprised. This Dart appears to have been an old-biddy-mobile/time machine that then sat for several years. Not a lot of maintenance was in evidence, which is a testament to the engineering & construction in these cars that it ran as well as it did. Motor flush was... disgusting.