I also agree... any soft material placed between the engine mount and the frame will cause more vibration instead of reducing it.
The best way to mount is directly to the frame and in a way that the engine will be very solid to the frame. Once mounted, the engine should not be able to be moved in the frame by pushing on the cylinder sideways.
You can put a thin layer of sealant on the mounting surfaces and tighten the engine down before the sealant starts to cure but putting any kind of soft material between the frame and engine mount's will cause more vibration and can cause more serious problems due to this excess vibration... U bolts can actually saw thru a frame tube when a rubber pad is placed between the mount bracket and frame.