Hello; I have fooled with anti-vibration mounts for years, with some success, and some failures. I tried what you mentioned on my Decathlon and a 70 OS. I found that if I tightened it up enough to keep from loosening, it didn't offer much vibe dampening. and if I used nylock nuts, so that I didn't have to cinch it down tight, the vibrations would eventually hog out the holes in the firewall. I had more success with "Lord mounts", these are bolts glued to rubber with the male threads on each end. They are available in all sizes I used dime sized ones. They were maybe 3/8 thick. They worked pretty good until they started to de-laminate (about two years) I think the oil and the heat reacted with the rubber and deteriated it. I have tried every anti-vibe mount known to man, and have now settled on the Dubro mount, as it is all that I can find here. It is expensive, but it works, and I think the benefits out-weigh the cost.