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Your problem sounds too familiar. I have a '79, 350 with a 4-speed. The engine always vibrated at 4,000 rpm, along with the rear view mirror, the shifter knob, the seats, the glass, basically the whole car. No matter what I did, I could not tune it to eliminate the vibration. The engine only had 53,000 miles on it, I bought it in 1985 with 10,000 miles and I really always maintained this car well, but the vibration was so bad, I wanted to drive the car off a cliff; I was at my wits end. One night while driving on the freeway, I was doing about 70 miles per hour, and the engine shook and then completely disintegrated.

In my rear view mirror, I saw a huge cloud of black smoke along with pieces of metal flying all over the place. With no power at the gas pedal, I coasted to the shoulder and looked under the car. There were five holes in the oil pan, one as large as a golf ball. Got a tow home (always use a flatbed tow truck, the sling type can mess up the front-end spoiler) and after removing the engine and tearing it apart, I found four connecting rods had exited through the oil pan, the camshaft had broken into seven pieces, two lifters and three pushrods were missing, and a couple of valves were bent. There were actually black molten metal parts in the bottom of the oil pan that were unidentifiable (most likely parts of the connecting rod caps). As it turned out, the crankshaft showed signs of excessive heat where the number 2 and 3 connecting rods attached and those two rods were the most mangled parts of the catastrophic event. I believe the initial problem may have been oil starvation to the rod bearings that perhaps spun within the rod and cap and wore out to the point of ultimate failure.
I would not rule out an internal problem with your engine. I checked everything on my car, the vibration dampener, clutch, transmission, drive shaft, half shafts, wheels, brakes and steering. The last place was the engine and that was it. One test that I did was shifting into neutral at 70 mph while on a downhill grade bringing the rpm’s down to 1,000. The
car had no vibration at that point but resumed when I shifted back in 4th gear. I should have deduced it was an engine problem, now I know. If your vibration persists, pull the engine and have it rebuilt, or better yet, get a new engine. They build them way better now than originally. I now have a crate 350/290 horse that passed smog, runs cleaner than ever before, and performes so well, I decided to keep the car and I drive it as often as I can.

Good luck to you!

Steve