This is chassis flex and the engine/ trans combo does not flex. Then something cracks.
X2, there is always some "racking" motion (like warping a playing card between your fingers) from the chassis - if not from engine torque, then from spring wrap on braking, or simply big one-wheel bumps. If the transmission is tied down too tightly, something breaks.
Poly mounts are safer and last longer, but transmit more vibration (whether it's enough to annoy you is a matter of personal taste). The driver's-side rubber mount can be kept alive by simply limiting the engine's rotational motion with a simply short length of wire cable (yes, you can use a hemi-jointed billet torque arm, if you're Donald Trump - but it's not better). One end is looped to a thimble and clamp and goes to a large bolt or bracket on the left side, the other to the chassis, with enough slack that normal engine vibration doesn't tension it. It has no effect at all (no noise, no vibration) unless the engine torques over completely to the point where the rubber mount is going to tear, then stops the motion. About $3.00 covers all the parts, infinitely adjustable, I've had one on my V8 street rod for 12 years and adjusted it once (you can even adjust it with the engine running).
