360 engine vibration
Yep I think so too. Whats your thought on the plug lead resistances?
The 2 different ranges of spark plug wire resistances are due to 2 different wire constructions/types. The latter, higher values are standard resistance wire, and the 5.7k ohms, etc. is in the normal range. The higher resistance ones would be used with standard plugs, but either can be made to work.
Damper as removed, is this the correct 360 LA damper?
The damper looks correct for a 360. The slots are, in effect, 'anti-weight'; i.e., the desired imbalanced weight is on the side opposite to the slots.
That old rubber, with the crack in it, has rendered the damper unable to 'damp' out the internal high frequency crank vibrations. (Those internal vibrations are in the 6-7 kHz range typically, and are not what you are feeling, but can damage/break a crankshaft if not damped out.) Whether that damper has anything to do with the overall vibration is another matter, but it needs replacing due to the age. And Oldkimmer is right... don't use an old damper....same problem: it won't damp properly once the rubber is hard/cracked.