I can tell you that never has the Auld Mug been held more dashingly by fewer people! (Okay, I'm no Churchill!)
Ron Butler has no website, and I can't seem to find his e-mail address. However, next time I call him, I'll ask about motor mounts. He errs on the side of caution---he was Ken Miles personal mechanic but also race-car development engineer (we shared a lasugh ovcer that--he essentially took split butt-welds, etc. and built in triagular gussets (?) maybe better called "braces". He eventually made the frames stonger and stronger, bit by bit. He ended up being called "boiler-plate" for structural overkill.
If I get any news or recommendations from him, I'll e-mail 'em to you. One problem with ANY flexible engine mount material is that SOMETHING has to pay the piper. If you're lucky its exhaust mounts, etc. if not so lucky, fuel and
oil lines etc. I have preferred hard-mount but only had one car with such a rig---of all things an old AH Sprite, the roll-cage assembly of which was heavier than the original car!
The original owner had removed all the e-mount rubber bushings. He had aluminum "biscuits" in shock-mount areas and in the anti-rool bars (added on, I believe). With a decent race seat (the other was almost a burlap bag!), the bumps weren't too bad. Body roll=zero, steering was quicker, and vibrations more evident. Not a car for long road trips.
G'luck again!