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BEFORE Gary's lathe work.
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And then:
1/7/15 -- Grant,
I believe flex-plates are supposed to flex. When bolting the trans to the flex-plate in cars, they flex. This keeps things in tension, and is a good thing. I've done a lot of installing, and all are that way.
Jim

1/7/15 -- Jim, OK, thanks, I'll follow up.
The one we’re replacing was "flat" against the flywheel, and the ring of rivets in the new one was only about 1/16" larger in diameter than the opening in the flywheel's center. So we thought the rivet heads should fit within that opening, hence the tiny cut made to let the plate lie flat. I'll re-look at the old setup to confirm (or not) that fit.
Really appreciate your observations.
I'll get back with you.
-=Grant=-

1/7/15 -- Jim, just returned from the boatyard (garage) where I examined the old (original?) flex plate, new flex-plate and flywheel. The OD of the ring of rivets in the old plate is quite a bit smaller (3/16") than the ring of rivets in the new plate. i.e., the plate lies flat against flywheel with lots of slop -- it can be moved around before the rivet heads interfere with the hole in the flywheel's center. For more than maybe you want to know, see my blathering here.

Old plate on left, new on right.
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Although I don't know if the old one was correct (I know it's not the original engine), the old one fit flat against the flywheel and the boat was running fine in that condition. I'll do some more follow-up.
-=Grant=-