I cheated a bit by scoring a set of "pre-drilled" H2b cases. The kind that were meant to have rubbermounts, but instead received the aluminum slugs fitted instead.

The factory holes are drilled to roughly 25mm, which is what Joop specifies as well. The Waterbuffalo silentblocks fit snug, but not too snug, where the "fingers" of the rubbermounts would be collapsed...so 25mm seems to be perfect.

All of the late H2 cases are drillable with plenty of meat.  It is a pretty straight forward deal after you have the holes drilled. The only issues that I had (which were minor) was that I had to file some aluminum off of the upper front case lugs at a 45 degree angle to give the cases room for clearance in the KH500 frame. I also had to shave an 1/8" slice of aluminum from the outside of the right lower case boss so the rubbermount would fit within the frame rails without being stressed. (It would have fit if I had JAMMED it in there, but it would have been too tight, and probably stressed the frame) If you look close at this photo, you can see that the entire outside of the engine case mount has been shaved off to fit the rubbermount within the frame.

Also make sure that you make correct length "spacer sleeves" to fit between all of the silentblocks. I made mine from galvanized electrical conduit from the local Hardware store. Perfect size and I.D. for the stock engine bolts. Once again, they must fit snug between the frame mounts, but not too tight as to stress the frame.
Here is a photo of the Suzuki mounts just sitting on top of the top rear case lug. The steel sleeve now sits between the rubbermounts.

and one last pic of the lower right rear mount..


IMHO, This is the best mod that you can do to an H2 motor for everyday street riding. It is a night and day difference over the stock vibration monster. Reeds come in a VERY close second.