Two things come to mind, check to make certain the damper is in good shape, and all the bolts on the flywheel/flex plate are tight and in place.

You can try disconnecting the vacuum advance to the distributor and see if the engine vibration goes away.  I’m assuming this is a ‘shutter’ or vibration that only occurs when the engine is in neutral and is being revved up?  If the vibration goes away with the vacuum advance disconnected, then it’s simply too much advance without any load on the engine.  If unhooking the distributor vacuum advance doesn't alleviate the problem, then disconnect all the front belts from the lower crankshaft pulley and see if that makes a difference.

You've had you leads off recently chasing that vacuum leak check your firing order.

cheers

Warren

Nat.  Something that comes to mind is a worn valve guide but your vacuum check should have singled that out.  When you ran the vacuum check, did you check the vacuum at the rpm point that the engine had the vibration?

Beyond that, I’ll suggest backing up the ignition timing at least 10° and see if the ‘shutter’ diminishes or goes away.  If this makes a difference, then concentrate on the ignition advance curve as the source of the problem.  If no change, then it’s back to the carburetor assuming a cranking compression check on the engine has all the cylinders within 10% of each other.

Harmonic dampers are designed specifically to counteract those shock waves (harmonics) being transmitted through the crankshaft.  That’s why they are called harmonic dampers and not harmonic balancers.  The dampers really don’t have anything to do with engine balance as long at the damper itself is undamaged and has the factory balance intact.  If the damper rings slips, then that could create an imbalance situation simply due to the outer damper being out of sync with the inner hub.

That leads into this.  The 1954 car 239 engines and the pickup and truck 239, 256, and 272 engines used solid hubs at the crankshaft without dampening capabilities.  Engine life is simply prolonged with an elastometer style of damper and especially when the engines get larger in displacement and have higher compression ratios.  The compression ratios of the first year Y's and the pickup engines was low enough that the factory engineers did not see the requirement for a damper on those particular engines.  As the compression ratio goes up, so does the need for a heavier harmonic damper.

Some additional checks best done one at a time:

Check and record your cold cranking compression in each cylinder. Should not differ by more than 5 percent if all are healthy.

Check convertor flexplate for damage or loose convertor or flywheel bolts.

Install a brand new set of plugs and check for vibration.

If you have the stock Ford Y-block distributor, static time the engine using a 12 volt check light (a 12 Volt light socket, bulb and two arm length leads with alligator clips). Ground one and attach other to wire lead at the coil post common to the distributor lead. Pull distributor coil wire so engine can not fire for safety. Turn ignition on. Turn engine clockwise by hand at the crank damper bolt till #1 is coming up. Engine would have fired #1 cylinder when bulb lights. Note your timing mark on vibration damper.  Mark #1 spark post position on distributor base. Remove distributor cap. Check rotor movement between zero advance and full mechanical plus vacuum advance to insure the distributor post at rest falls midway between the full rotor tip movement range. This insures cap and rotor have been properly clocked by your favorite parts stores secret offshore supplier (and you thought they were usa made) and that spark will not jump to wrong post at full vacuum advance.

Check inside dome of distributor cap for any kind of dust or grease residue and any sign of electrical tracking between all nine posts. Replace at hint of a problem.

In total darkness. have someone hold engine at peak vibration rpm and check engine plug wires for any visible sign of electrical leakage to ground or adjacent wires. Y-Blocks hide plug wires very well. Get a new spark plug wire. Bypass each plug wire one at a time to see if the new wire eliminates the vibration.

Try a different condenser and check for vibration. They can go bad. I found the hold down screw loose on a condenser once. Drove me nuts finding it.

Try a different coil and check for vibration.

If power brake equipped, check for booster diaphram leak by plugging intake manifold tap for booster and check for vibration.

#8 cylinder, lighter in color, could mean burning leaner (intake manifold gasket leak) or coolant getting into that cylinder. Any radiator coolant loss over 6 months?

If none of the prior tests fix the problem, turn focus on carburetor. Let us know what carburetor list number you have. I ran my Holley list 1273 50 years with a slight hesitation off idle that I now attribute to insufficant transition slot exposure. Vanished when 650 cfm 4011 replaced it.  

Jerome