Congrats on finding and resolving the problem. Did you take a real good look at the collar of the flexplate? I mean under magnification. If that was loose for any amount of time there's a chance of cracks (invisible to the naked eye) that will cause you all kinds of grief down the road. This is especially true if you are going to twist that engine tight. Just my opinion, but if you have that certified flywheel sitting there I'd be swapping it in and trash the one that was loose.

With the nice weather I hope you rewarded the car for starting and running great with a nice shampoo, rinse and massage...

Edit: (just saw your other question)

A flexplate for an externally balanced engine will have weights welded to it just inside the ring gear. On an internally balanced engine the flexplate will not have these weights.