Forgive the long post. So I have been chasing a vibration in my car since the day I bought it a few years ago and I think I am losing my mind. The day I bought the car was raining and I didn't do a very long test drive which was super bad on me. Anyway, Once I got it home and got to drive it I noticed this vibration and have since sunk a lot of time and a lot of money trying to chase it down. I am at my wit's end and looking for some help. The car has a 350 in that was rebuilt about 7000 miles ago. It was installed by a prior owner and built by a local shop but didn't come with any info on the build. it has a 650 street avenger holly and MSD ignition mated to an M22 Muncie 4 speed. The vibration presented at around 2.5-3k and went away at 3500 RPM which happens to be right in my cruising speed of 60-65 indicated, so super annoying. This vibration is there with the clutch in or out and felt in every gear as I go thru them but most noticeable in 3 and 4 but with a close ration M22 you don't spend a lot of time in that RPM range. When on the road it shakes the car both up and down in that RPM range but is smooth as glass on either side. When I run the car up to the RPM range in the garage I feel it but it's not as noticeable. My first thought was that it was speed related as that's when I noticed it at first. So one of the first things I did was have all the wheels checked out and rebalanced and that did help a small amount. Tires are old and need replacing but that doesn't seem to be the issue as I can push the clutch in and drop the RPMs at speed and the vibration goes away completely. Next, I had the ring and pinion changed out along with new U-Joints and rebalanced driveshaft as it was clear that the ring and pinion was shot, I thought well that has to be the cause. And again I was disappointed as it was a little better but that vibration was still there. About this time I had figured out that it wasn't speed related but RPM and really started to dig into possibilities and solutions, looking over dozens of other peoples post about their vibration issues hoping to find a reason for mine. I started by trying some easier things like changing out the old motor mounts to the correct ones and that seemed to help but that cursed vibration was still there. I pulled the belt's off and ran it and no change. I pulled plug wires one at a time at RPM to check if I had a misfire or something and it wasn't that either. I checked all the front in steering components and they all look good. I was super frustrated at that point and wanted to sell the car and then found the rear subframe rails needed some attention and wouldn't sell the car like that. So I had both rear subframes repaired and added new springs, plus all new hardware and new shocks all the way around (Hotchkis). That certainly helped the ride and seemed to help the vibration a little but it was still there. One odd thing that would happen was every once in a while that vibration seemed to disappear completely for a few hours on a drive and then would come right back the next time I drove it. It didn't happen all the time but often enough that I was perplexed at what could be happing that would cause that to come and go like that. It was never for long and I kind of convinced myself it was me just becoming used to it and not noticing it as much. I finally had enough and had the transmission pulled by a local shop and found a centerforce clutch in there that had the weights shifted. I had read so many posts about people chasing vibrations that I knew that was a known issue and so when we found that I was positive that had to be it. So they pulled the flywheel and had it resurfaced, checked the pilot bushing and that seemed ok then installed a new RAM clutch w/ new throwout bearing and put it all back together. At the same time, I had also all the body bushing replaced (Solid) and the front subframe mounting holes in the frames fixed as there was rust present and the holes were too big. Drove the car for a short spin after the clutch replacment and the vibration was gone at 3-3500 and I was elated. Had some headers installed at the same time so was waiting on the new exhaust system to be installed before I really test drove it. Well once the exhaust was in I finally got to take it on a proper test ride and it was awesome until I got above around 4000 RPM and that vibration was back with a vengeance tapering off around 4500 -5000 rpm just as before only at 1000 RPM higher. crappy Video I know but you can see and hear the vibration when I go over 4k and the car smoothes out at 5k. It's really noticeable when I drop it back down and cruise at 4k as the vibration is gone and the car is super smooth. (.) We are now thinking about trying to balance the flywheel to the pressure plate but I am looking for any other suggestions on what could be causing this as I am out of ideas and if we drop the trans again I don't want to leave any stones unturned. I was thinking about a new Flywheel as the original is still in the car? Pilot bushing or starting to dig into the engine rotating assembly and balance as a possibility. But that is one that I am not sure I would bother with as I do have the numbers matching block I plan on rebuilding at some point. Anyway, I am very open to suggestions at this point so let me know if you have had a similar situation you were able to resolve as I am willing to try almost anything at this point. One bright spot is the car drives and handles amazingly at this point and I am not sure I would have gone through so much so fast if I wasn't chasing this vibration so I guess that's a positive.