I did consider pulling the spark plugs, so I didnt have to fight against the compression stroke. Fortunately I had a three foot breaker bar, no problem. With a 3 foot breaker, anyone can generate some serious torque.
The plan was to keep unbolting, rotating, and rebolting the flexplate to the next position on the converter, until the vibration stopped. With all the cross-torquing, it was possible I'd have to bar the engine dozen of times looking for the sweet spot.
I lucked out and it ran smooth after one 45° clockwise rotation. Phew!
If the plugs were out for barring the engine, and it vibrated again after the plugs were back in, it would have gotten old, taking them out over and over for each new rotation. Good thing the harmonic balancer bolt is hardened as hell, so it's none the worse for wear.