Another option is a poly mount made by Craig Aylsworth. I used them in my SPF with a Shelby block, and there is little vibration, and no chance of header slap on the body. Very stout, and very clean installation. Much better than modifying a rubber mount.
Here's a photo, and Craig's e-mail. The cost for mine was $283 + shipping, but well worth it.
craigayl@comcast.net
Installation was a snap, and the alignment bolts made it easy to install, with no worries of misalinged bolts or damaged threads.
Miles