I also installed the Olthoff custom urethane motor mounts on my SPF, after almost 2 years of using rubber mounts (I took Hal's advice shortly after he had his changed over!).
I am very happy with mine also. They look great, and there is no noticeable increase in vibration (still using a rubber trans mount). There is no more "rocking" of my engine under hard acceleration as I had with the Ford rubber mounts (the top of the left pipe and the bottom of the right pipe showed marks from hitting the body cutout when I was using the rubber mounts).
You can contact the Olthoffs at their website or get phone contact information at
http://www.olthoff-racing.com/
They are a pleasure to do business with, and are a part of Cobra history.
take care,
Jim Kellogg SPF887