Ok so I know this kind of question has been posted before but my case is confusing me.

I have a 318 engine recently built but not by me, and a 904 transmission that I thought was bolted to another 318. But the converter is obviously externally balanced. I am getting conflicting info on if all 318 are internally balanced. If this is so it means my torque converter was off a 360, right?

I have been having a really bad vibration issue that is most noticeable at idol in park. From what I understand this means that it should be an engine issue, right? But I have gone through the basics of motor mounts, timing, and a power balance test. It still vibrates bad like it’s running on 7 or even 6 cyl. The only thing i haven't checked is idol fuel mixture. But i was seeing a clean 100rpm drop on a power balance so i didn't bother. (maybe that was dumb) The transmission has gone south so its hard for me to rely on driving symptoms. Before it did, i can say that at freeway speeds i didn't feel a vibration. If the TC was wrong shouldn't vibrations get worse?

Is this a motor or transmission vibration problem? Or is it possible i have the wrong TC and a different cause of vibration?
Help......