Polyurethane is much denser than rubber. The Schumacher hemi to b kmember mounts have small diameter bushings which will not cushion the vibrations especially with more cam in a motor. We found the Schumacher mounts also raise the motor higher than stock which changes the driveline angle. Add a Poly trans mount and you amplify the vibrations more.
I run rubber mounts whenever I can on street cars!

Brad