Engine vibration at 40 mph and higher

Hello. I own a 1951 Chevrolet Styleline DeLuxe 4 door passenger sedan with a 216.5 cu. In. (3.6 litre) inline 6 cylinder valve-in-head engine. The car has about 153,000 miles on it. The engine vibrates steadily with a smooth humming vibration at about 40 mph to 50 mph. Over 50 mph the vibration starts to become a deep rumble, definitely at crankshaft speed. The clutch pedal vibrates slightly, and the throwout bearing chatters lightly, too. I am wondering if this is from an out-of-balance flywheel. At about 100,000 I rebuilt necessary parts of the eninge. Original pistons were cleaned and re-used, since they are perfect, and the cylinders had less than.003 inches of taper. New rings and bearings and valves, guides, oil pump, and seals were installed. Appropriate machining was performed. The machine shop I had do the crank machining recommended balancing the crank and pistons. I didn't see how this was necessary. The engine was always a smooth running engine, and it says clearly, several times, in the Chevrolet shop manual that the engine is precision balanced at the factory as a unit (assembled). At the time I did not know that this included the flywheel.
After I got the engine all back together, it was quite smooth, but after a long 200 mile run, the humming and vibration started. I opened the engine to see if I could find trouble. I found that the rear bearing had started to fail. I had the rear bearing journal dressed and checked, and installed a new bearing, which doesn't seem to have troubles. I try not to take the car over 55, though it was originally quite capable of going 99 mph (not that I ever sustained speeds over 65). I am wondering if my not having the flywheel balanced with the crankshaft and pistons is what is causing this vibration, since they were all originally in balance as a unit. With the crankshaft now being balanced as its own unit (including pistons), I feel that it is now out of balance with the flywheel. Would balancing the flywheel and pressure plate as its own unit now correct the trouble? I really don't care to remove the crankshaft again.
I also wonder if this vibration is robbing power and mileage. The car has very good acceleration at lower speeds, but the power tends to be a bit lacking at higher speeds, and gets worse after a long run (everything else is well adjusted--valves, heat riser (which can severely rob power if intake overheats), carburetor, timing, and all the other things that can rob power after a long run). I used to always be able to get 18-20 mpg with the car prior to the vibration, which is typical mileage for this car. Now I usually get 16, and sometimes worse. Only rarely do I get 18 mpg. The carburetor is well adjusted and well jetted, and the engine does not misfire as far as I can tell, though I have noticed that one spark plug is a shade ever so slightly more brown than the rest, which are, only after much driving, a very light shade of tan or gray. I don't feel that the vibration is from misfiring or anything in the fuel/ignition department. Valves seal well, and compression is good. All valve springs seem to function well, and were shimmed after the rebuild. All valves have good clearance--.015 inches for exhaust, and.008 for intake, after engine is thoroughly warm and normalized.

What causes the vibration?

Thank you much!

Thomas