I too will be doing the mustache bar bushings soon. I don't have a clunk problem (new diff mount and new trans mount), but I do have a vibration that I am trying to eliminate. I will be replacing the trans crossmember bushings soon. I bought Nissan rubber bushings and already installed them into a spare crossmember, so it will be a quick crossmember swap. My vibration is under acceleration in lower gears. If I am going even slightly downhill (i.e. highway onramp) it is completely smooth, but flat ground or any incline brings out the problem. Engine, trans and diff mounts except the mustache bar bushings are new, so hopefully they solve the problem. If not, it might be my diff, I guess. I did buy urethane mustache bushings, so hopefully they don't transmit too much more noise than the old, worn out rubber ones.

A few weeks ago, there were several days of rain in the forcast, so I had a good window to finally replace the mustache bar bushings in my Z. I had already replaced the trans crossmember bushings, but my vibration was still there. I got the new poly bushings installed, but while I had the car on jack stands, I went ahead and pulled the suspension apart so I could trim the fender lips to fix my tire rub. Unfortunately, I stretched a braided brake line too much and pulled the end off, so I had to wait a week for parts to come from MSA. I got the new hoses Monday and got everything back together and bled.

YAY! My vibration is gone. Replacing the mustache bar bushings did the trick and now my Z finally accelerates smoothly through the gears. I did, however, pick up additional gear noise from the poly bushings. It isn't terrible, but is louder than before. I will take the noise over the vibration any day though!

For pictures of my fender trimming, check this thread

http://www.classiczcars.com/forums/showthread.php?t=34282&page=2