Hi JohnHi Bob
The phrase "never assume" springs to mind. Even the original used a spring type mounting, surprised that your kit was hard mounted! maybe something was missed out, as all the different conversions I've seen have had some sort of flexible mount. Suggest you get back on to your supplier.
it did come with a flex mount the type that has a bonded stud either side of a rubber doughnut which was far to rigid so Paul who has been doing this for me made a completely new rear support using the original spring drove it this afternoon and its vibration free so its now gone for some paintwork and my christmas will be spent replacing all the chrome i have removed.I have had a couple of classics both cars and motor bikes i have always joined the clubs associated with them and found there forums very helpful since joining JEC i can say that the response i have had to this problem is excellent
Many thanks Bob