vivivivivibration

If you order in a new 318/340 neutral balance flywheel you should be able to see the difference between what you have and the new Neutral Balance one.

On the thought of your long bolt idea in the flywheel. Could try a simple test like this to prove all of this is true.

Take the bolt out of the pressure plate closest to the Large balancing drill hole on the flywheel put in a 1/4 inch longer bolt with 4 flat washers stacked on top of the pressure plate to offset the missing weight in the flywheel and bolt it down tight. Roll it over by hand a couple of revolutions to make sure there is plenty of clearance on everything and will not catch on something.

Then give it a quick start (only as a test, do not drive it down the road like this) and see if your vibration problem starts to go away. If so then you know you need to swap out your flywheel.