I too have heard about using some type of Harley part in lieu of the stock Norton oil tank mounts. However, I don’t have a description or part number for the Harley bits. But, someone on this forum must have it?
Make sure your isos are not too tight. Isos that are adjusted for maximum handling performance can cause some serious vibration at the oil tank. This vibration will cause fatigue and failure of the rubber mounts and / or metal brackets on the tank.
I have used the Harley mounts for years. I don't know of a part number, but I believe they were used on the shovel heads oil tank. I get mine at the local swap meet (or Walneck's meets since they are almost all Harley now). Most of the bigger vendors have a box with a couple dozen on display. They look like small Norton muffler mount, with a 1/4-20 stud on each end. I usually cut a little length off each end of the stud for easier mounting. They have been on my '73 for probably 20 years. I have had no problems with the brackets breaking either, but I agree that this is probably caused by the iso adjustment. Too loose or too tight could cause a problem.
Before cutting the stud, be sure to have a nut threaded on and all the way down.
Then when you finish cutting you can back off the nut and it will "cut" the threads at the end to allow for easily threading on a new nut. This works well on smaller studs that can be a pain to get a nut on after a cut.
I have found that the original rubber mounts were a little too hard, and the newer ones offer more flex.
Nothing like fresh rubber.