360 engine vibration

Different deck heights, perhaps. Can effect compression and/or valve opening/closing which effects compression readings if this has hydraulic lifters and the pushrod length is such that the lifters are near the edge of their travel range. My son and I got a 5 psi average difference between left and right banks even with careful setting of deck heights.

I did not want to say it before and be a wet blanket, but I am not surprised on the damper not fixing the vibration. 10-15 psi difference in left and right banks ought not to cause any noticeable vibration... the firing sequence is not left-right-left-right all the time. So I don't think this is your issue.

BTW, the idea that the crank is properly balanced is debatable..... that depends on the pistons, rods, etc..... which seem to be unknowns at this point.

EDIT TO ADD: Those compression numbers are NOWHERE near to stock numbers. So it seems pretty certain that the pistons are not stock. That leads to questions on overall balance of the engine.