318 running rough and losing vacuum when in gear.

That nasty ratcheting sound when going back to park is normal because the output shaft is spinning.
They are made so that excessive force against the park lock will result in the park lock giving instead of blowing the side of the case out.
It's not something you want to do on a regular basis of course, but it's made to do exactly what was described.
If I were the owner of that car I'd find out where the problem with the ignition, timing and carburetion is because I'd bet 100 bucks it isn't because of the converter.
It sounds like it's mostly a carb problem.