727 flex plate?

mystery deepens......
After removing the trans to replace the flex plate, I discover the flex plate plate was modified to fit the crankshaft (see pics).
So after doing some research and test fitting to multiple cranks, I find out that my mystery motor has a 360 crankshaft (external balanced?) in a 318 block. So now the question is, what type of flex plate to get, since there are no weights on converter (like an external motor would) and a 360 cast crank?

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