Externally balanced converters will have large weights on them and flywheels usually have cutouts.
There are some externally balanced aftermarket flexplates which will have a weight added or a cutout.
Sounds like good questions for the fella I bought it from to see if he realized all that when the motor and tranny rebuilds were going on. I’m thinking this has been a long term issue with this car back when the original drive train was removed and replaced with a J400 (73). I haven’t checked the tranny yet.
