My engine rocks and shakes the whole car!! Need Suggestions!!

Howdy all, cosgig here. I have had a longstanding problem with my GTX now for going on 15 years, and it's buggin' the heck out of me!! Heres the details...

When I got the car the motor wasn't running due to a spun bearing, so I don't know how it was running before I pulled the motor. The motor is the original 440, which I had out and rebuilt in 1996 or so. It has about 5000 miles since the rebuild. It is rebuilt to standard specs and is standard issue 67 stuff. I replaced the motor mount insulators with new OE rubber ones as well as the trans mount.

Ever since getting it running back in '96, the motor rocks at idle and shakes the entire car. When sitting at a light under brake idle, it really rocks the car. Off idle, no problems, and the car runs like a raped ape all through the throttle, and pulls strong. I have other cars with the rubber insulator motor mounts, so I can't believe that this car should be so much more violent on them.

Why am I worried about it after so long?? Well, the car started to develop a "clunking" sound under acceleration while under load, which I have diagnosed as the drivers side exhaust pipe banging on the crossmember cutout. Now, I wouldn't normally put the 2 issues together except that it looks like my pipes are all skewed under the car. I have a very solid mounted system with an "X" pipe and it is very well tied together. The drivers side is way up closer to the clearance areas and the passenger side has clearance to sell!! Could the motor rocking be having an effect on this, loosening and tweaking the drivers side?? I'm trying to figure that out!! All the mounts are tight, the rubber still looks good and is not cracking, so I'm at a loss here!!

It's time to remedy this once and for all, so....

Is there a polyurethane motor mount that will help this?? Is there some internal maladie that I should be looking into here?? HELP!!!!

Any input appreciated!! Thanks FABO, y'all ROCK!!! (yep, more than my motor!!) Geof