It got a new chain, gears, lifters bearings, oil pump. Cam & crank were polished. Pistons reused. The flex plate was replaced at the time of rebuild. I find it unlikely that it is the wrong one....I could be wrong.....worth checking.
This issue started after the engine was removed for a rebuild. Just a stock rebuild, nothing fancy. The engine has always seemed to have a long crank before it fires but it was doing that before the rebuild. So the problem is a miss at idle that shakes the engine, its not a dead miss as it comes...