Vibrations on a 3VC engine

[Bozo]

My 89 3Cv Fj1200 has some harsher than normal vibration at the usual 3000 to 3800rpm. My engine is not standard (cams, pistons exhaust etc), my standard 89 FJ has a milder form of vibration so is not that much of a problem.
I was thinking of adapting a similar down tube and mounts from a 1991 Fj. (I like my 89 so trading it in for a smoother 91 is not an option), I also know the back mounts are different but I believe its the front that is causing the vibration.
My question is, has anyone done something similar?

Before you ask eliminate -
New chain and sprockets
Shims and carbies checked
New plugs
All mounts are tensioned
All rubber mounts in good condition.
When holding revs at around 3500rpm in neutral you can feel the engine vibrating