Stiffer mounts will just put the vibration into the chassis and make it vibrate in addition to the engine.
As to the CF clutch;
they have flyweights to assist with clamping at higher rpm. The weights fly away from the center with rpm, and a weakazz spring is supposed to return them afterward... but they don't always ALL return to the SAME spot.... so then you get a mild vibration. I cure mine with a neutral-blip at speed. At idle, it sometimes is persistent, but since I know what it is, I ignore it. Plus it only occasionally does it, and it is never intermittent, nor anywhere near the magnitude of yours. Even when my 340 disc spits out it's springs, the magnitude of vibration is nowhere near where yours is.
To me; your vibration is a either;
excessive timing
wandering timing,
or a carburetion issue,(like a fluctuating fuel level, or a left right low-speed imbalance, or contaminated fuel )
AND
coupled with something else I haven't been able to figure out,and because I can't, it leads me to believe it's engine balance.
The clue for me is the way it alternates; there seem to be two or more distinct vibrations, that tend to combine and then to cancel at periodic and somewhat regular intervals. I have never seen it in the tune. Well at least not in a V8 lol.
IMO,
if this is crank-balance related, you need to fix this before it wrecks your bearings, or worse.
If it is ignition timing related, It is imperative to fix it, as it can lead to broken parts very quickly.
If it is fuel, then probably it is just annoying, unless it extends into WOT situations.
If it's mechanical, like uneven compression, a bent rod(s), or such like; it would just be terribly annoying to me, until something broke, and then there you are calling the towtuck.
BTW, if you have powerbrakes, and no one has mentioned it;check the fluid level in the m/c.
