Re: Secure mounting.
There's nothing wrong with rubber mounting an engine as long as it's done right. Using a rubber spacer between contact points and a bolt straight through sandwiching the rubber is not the right way. You will have problems with the bolts tension tightening and loosing with the cycling vibrations and possibly fatigue cracking your hardware and mounts or you cult have problems keeping them torqued. For these engines I can't see this being a serious issue if regular torqueing of the hardware is kept up on.
I'm not sure I agree with the comment that it will add more violent vibrations to an engine that you just won't feel. I can see where your going with this. The engine will vibrate and rock in the mounts further than a solid mount but would the rubber not dampen the impact rather than things being solid with less motion but no damping?
If your looking to rubber mount an engine to last the best way is to use rubber bushing where the bolt can go through and torque tight against a solid mount and have the rubber isolating that entire assembly. Like a transmission mount on a car or have a look at the 2003 and earlier solid mount harley engines vs the 2004 up. Almost the exact same engine other than provisions for rubber mounts.