From initial description of problem, I would hope that it's not an imbalance issue afterall, after all this distrib mayhem. You're too far away, as I've had to pinpoint many probs over the years for many, and would love to 2 cents it. When I worked at Chev dealer, we installed a reman short block in a suburban IIRC, rebuilt from an outside builder and thing had weird vibration, so slight, but the customer was not liking or accepting it. Pulled the belts, pushed converter back in trans to eliminate them as being issue. The builder had us swap to new mounts(origs were still good) and there was no change. They ended up having a company come out to balance it in chassis with some fancy harmonic locating gizmo setup.