I also am starting to think that the problem has been there since the start and I just did not notice it. It is the matching numbers 383 block. It is the crankshaft that was in the engine and I believe it is original, although when we tore it down for the rebuild a few years ago, it was obvious that it had been gone through before, so anything is possible. It is a forged crankshaft stock stroke for a 383. The shop that did the machine work for me balanced the rotating assembly. They had to take material out of the counterweights to get it to balance with the lighter pistons that I used. I did not get any data with the balance job, just a note on the bill that it was balanced. The shop is very respected and reliable, so it is hard to believe that the balance is off, but mistakes do happen.

At the beginning of the search for the source of my problem I did a RPM drop test for each cylinder by pulling each plug wire one by one. Cylinders #3 and #5 did not drop as much as the others. They were not dead, just a bit less. IIRC they were about 80 RPM and the others were around 100 RPM. I'm going to repeat this test and verify the previous results. Also going to try again to verify that the vibration is not the result of a misfire. I did pull and inspect all the plugs and did not notice anything that would suggest a misfire, but will do that again. The vibration does not feel like a misfire at least based on my experience. I don't feel or see the vibration in the engine like I do in the steering wheel and through the seats.

Yesterday I found an issue with the left motor mount insulator that I thought might be the problem. The stud on the aftermarket insulator was too long and the end of it was gouging slightly into the block. The right side had clearance. I found an original insulator in my parts stash that was in OK shape and also the rubber seemed much softer, so I put that in place to see if it made any difference. There was no noticeable change in the vibration.

Thanks again to everyone that has provided their thoughts and experience to help me figure out this problem.

Regards,

John