Good points above about checking the engine and gearbox mounting points.
If you’re happy that it’s not that, then there’s two ways an engine may feel ‘out of balance’, one is a physical imbalance the other is an imbalance of combustion.
Looking at physical, I’d start by removing the fan belt. I know you checked the fan span freely before, but worth a double check. You will be ok to run the engine briefly with the belt removed.
You previously said the problem showed up when revving in neutral and stationary, so you should be able to back to back it with the belt on then off and see if that makes a difference to the vibrations.
Removing the lower flywheel cover, I’d then take a look at any sign of damage, or play/movement in the clutch pressure plate or flywheel. I’d also give the front pulley a good firm pull / push checking for any movement.
If that all seems normal then if you still think it’s a physical imbalance then you’re talking about pulling the engine and doing a full strip down to check for mechanical damage to the pistons/rods/crank/cam and associated bearings.
You are suffering with fouling to one spark plug and not the other, which could indicate sone sort of mechanical imbalance, damage to position rings/bore on one cylinder, A bent valve that’s worn the guide and many other potential issues.
Don’t panic though, as the difference in plugs could also be a combustion imbalance.
So before going that far and stripping the engine down, I’d backtrack to the potential combustion imbalance. The Siamese intake port design rules out a lot of the factors on the intake side, and your recent fairly balanced compression test numbers also rule out a reason for imbalanced combustion.
Still, I think it would be worth checking a couple of things. If you have a dial gauge, or even it might be possible with calipers if you’re careful, check that all 4 valves are opening with equal lift. A worn cam lobe would allow different amounts of gas to flow in/out of the two cylinders, and might not show on a compression test. While you’re there look for any mechanical damage like a broken valve spring, or bent pushrod.
You could remove the exhaust and check for any signs of internal blockage. If one pipe was flowing better than the other that would create imbalanced combustion.
That kinda covers it for air flow imbalance, the final factor could still be a spark power imbalance. Even though you’re running a wasted spark system which again removes some of the chance for imbalance, there are two lobes on the cam in the distributer that open the points. Check both sides for wear. Check the points gap, then rotate the crank 360 degrees and check the points gap again, now on the other side of the distributer cam. A difference in gap will mean a difference in dwell, and therefore spark power, and spark advance between the two cylinders.
If you’ve not found the smoking gun by this point, then maybe it’s time for that full engine strip down to see if you can find the problem.
. Sorry I'm probably not the best person to help, as I know little of the 124 spider. Rather than further clutter up Smart51's thread in the classic 500 section, you'll likely find the people who know your car better in the Classic 124 section; 
