I've seen instances where the poly mount could damage the transmission case from everything moving and the tailshaft being held too stiff in place cracks... Only reason why I see sense in poly motor mounts is because they don't vibe or make as much noise as solid mounts and they keep the motor nice and stiff at the proper drivetrain angle in height, so there is no sag like the rubber ones do resulting in your y pipe not slapping the shit out of your floor when the motor is under torque and twists and makes the y pipe hit. This is why I wish I did poly motor mounts when I did the headers because everything looked like it cleared good but I didn't take into account that the motor twists a lot more with worn out rubber mounts in there.
So in conclusion I'd say get poly inserts like I'm gonna do, drill out the rivets holding the metal mount brackets together and put the poly ones in before you bolt in your headers and y pipe so you don't end up like me with banging pipes making my knock sensor go crazy. >.< Then do what I did get a cheap 10 dollar rubber tranny mount and use some 3M window adhesive and fill up the hole in the rubber with it, after that I never broke one again and its still rubber so no worrying about breaking stuff
Its the hole thru the rubber that weakens them and makes them tear combined with oil soakage
