1966 cuda new single res leaks

FWIW I had exactly the same experience with the same type of MC from Rockauto. The supplied fitting for the bottom plug wasn’t even correct type!
I eventually used the fittings from the old MC and it took a number of efforts to get the thing not to leak. Furthermore, the lid leaks no matter what.
I’ve decided to convert to dual mc and matched distribution block to get past my concerns about brake failure.