This has been gone over at least a trillion times in here and the proven mount type for these is solid. We see people in here all the time complaining about vibrations and Every time it was due to poor mounting with rubber where it doesn't belong.
Several other members in here including myself have Harleys and Buells as well as other brand large V twin cruisers and there's a huge difference between an 800 pound Harley and a 50 pound bicycle, but the reason Harley switched over to rubber isolation mounts was when they began making counter balanced engines, All the EVO engines and older were solid mounted except for a few of the Dyna type frames.
You can mount these engines in Your bike however you wish, we will be there to help when you crack your frame and we'll also answer all your questions on how to get rid of these aweful vibrations. Take it from those of us who know... Solid is the way to go on these if you want a smooth ride and to keep the same frame for more than a few months. Like I said, there are just tons of posts on how to properly mount an engine, and it's no secret that rubber has no place on these mounts. The more solid and rigid these are mounted, the less vibes you'll feel at the bars and seat, and the longer your frame will last.
There are Very, and I mean Very few people in here that can come up with a successful isolation mount setup with these so to keep it simple and to help the guys who have never worked on a bicycle before let alone install an engine on one, the general concensus is to help the new guy who needs all the help he can just to get the kit installed and not confuse them with the "fab up this, weld here, cut here' etc, etc, etc...
Now if you can come up with a properly functioning isolation mount setup that's dead smooth and doesn't crack the frame in a month, please feel free to do a write up on it, but when helping the new guy out, we can't assume he has 40+ years experience building these, and anyway, it's been proven time and time again that mounting everything nice and rigid is very dependable and smooth.