Hi folks,
I thought I should tell you how this worked out, it's always good to get the end of the story
I put the flywheel on an old mechanical wheel balancing machine to find out how it's been balanced, and how much and also where. It turned out to be about 57 grams (2 oz) and IIRC that is about half of the 360" balance weight. It is also located differently, instead of being right where the drain plug is, it is located clockwise to the next hole where the flexplate bolts up. I fabbed two weights of 1 oz (one on each side of the bolt mount) and welded them in place. No vibrations!
I have no idea why it is like it is, but it works. The harmonic damper seems to be an "unbalanced" one.
After this summer I will install an A518 and then I'll know what to weld on.