I get asked this question a lot, and the answer is no. The transmission mount should never be stiffer than the engine mounts. I recommend using a stock transmission mount, as long as it’s in good shape, when paired with my motor mounts. A stiff transmission mount runs the high risk of cracking the transmission case. The motor mounts do (or should do) all of the work in resisting the twisting motion of the engine while the transmission mount is simply to vertically support the end of the transmission. If the transmission mount does not have any give then all of the twisting from the engine will be transferred to the frame via the transmission case which it was not designed to do. The factory transmission mount works just fine.