Unfortunately, there is not much friendly advice given on this website. It would seem that with so few Z's on the road that it would not be this way, but invariably most of the time newbie's are told to "search, its all been discussed before", which to me is really rude. Normally this turns off someone who is new to the Z scene. This might be why there are so few posts anymore here compared to a couple of years ago when I first started working on my Z.
I think that the main reason people post on this site is for the social experience of interaction with other people who own the same car. Otherwise what's the point of having the site? Yes, a lot of info has been discussed before, no doubt, and there is a huge collection of knowledge here with the group.
So with all that said, this will probably get moved to the "tool shed". As for what you posted about your vibration, it can be any number of things, including what you mentioned about the converter being out of balance. Another possibility would be that the motor might not be balanced, or balanced to a flywheel/clutch set-up. Do you know the history on who put the motor together? Check the angle on the driveshaft by disconnecting the transmission mounts and moving it up / down or left to right. Check and see what the angle is with a magnetic bubble angle prior to disconnection and then see if lowering it makes it straighter. The straighter the driveshaft is from the transmission to the rear-end the better. Plus the more the rear end pinion points towards the transmission the better. Verify that the driveshaft is balanced as well, could be the issue. Check the half shaft joints too, if they are worn out they can vibrate too. Hope you figure it out, start checking and figure it out by process of elimination. Good luck-