First, you need to separate whether it is an engine vibration or a drive train vib. When you go through the gears, is the vibration tagged to the RPMs in every gear or 3-4k in a particular gear? SInce its in every gear, I would rule out drive train angles, u joints etc. Since it appears to be by RPM, then start cheap. Inspect teh exhaust especially back at the gas tank for evidence of contact where ever it is close to any solid object. Since the exhaust is solidly connected to the motor, its gonna twist with the motor and well you get the picture. You can wedge a peice of wood in place to see if ends the noise. I run dual duals out the back and its tight next to the gas tank. Took me a while to rework the mount and frankly a BHF on the tank to get a little extra room to end that problem. Start at the back and work your way forward methodically inspecting with a mirror and good light on both sides as applicable. Don't forget the cross over and header clearance at the shock towers, clutch Z bar assembly and steering box.
Next, is the vib a function of torque? Does it happen at 3-4k regardless of how much throttle you are giving e.g. neutral throttle at 70 or only when on-throttle? If its a function of torque, could be a bad motor mount from too much HP for stock mounts. Once you rip a mount your done, needs replacement and of course, when you are torquing hard, one side will lift till something hits something. I used to have to chain my motor ~400hp to the frame from the head before I went to custom motor mounts, others "pin" the lift side of stock mounts. Could be once you had a good clutch, you did a burnout or power run enjoying the benefits of a clutch that finally works and ripped a motor mount with all your new found power. Put the car in gear with the hood open and let the clutch out while on the brakes heel-toing the throttle. If you see the motor lifting on one side - its a motor mount - it will be noticeable.
Finally, I would get it up on a lift with someone in the car and run it to replicate the noise whilst I was underneath with a BFScrewdriver wedged between the bone outside your ear and different places on the car - listening around to identify where the vibe is coming from. I do not like the fact that the thrust bearing was shot on a new motor. Are you sure you have the right pilot bearing and it wasn't farcled on installation? Loose INput bearing on the transmission...
Last edited by dobrostang; 02-23-2019 at 07:55 AM.