There was an article in RCM on how to soft mount any motor mount.
What you did was to have rubber grommets between the mount and the forewall and also behind the firewall, underneath a large washer and the nut.
Same applies to the mount side, place a rubber grommet on the bolt, under the head.
I would assume you need nylock nuts if you use this method, or at least some mechanicm to tighten the mount properly, yet so the nuts don't vibrate loose.

Hope this helps