Didn't read all the replies, but, some things to think about..
Gene is right on with the flywheel.
If that is true, it could be bad news.
My engine builder balanced the rotating mass together.
That means the crank, rods, pistons and flywheel and harmonic balancer. He did drill the flywheel some.
its the pressure plate out of balance, which is a long shot,
or the bushing in the end of the crank for the trans pilot shaft is missing or the wrong size.
If the flywheel was not bolted squarely with the crank flange, it would vibrate like crazy.
If the pressure plate were not fitting flat on the flywheel, it would vibrate like crazy.
If the bushing in the end of the tail stock for the drive shaft were wrong, worn out or missing, it would allow the driveshaft to run out.
Check your tranny mount for excessive play. you should have put a new one in there.
Do you have the wrong harmonic balancer on there?
When you find the problem, please post the solution.
Thanks for all the info. A lot to consider. I took driveshaft and had it balanced. He said it was definitely out. I reinstalled and now it runs smooth tillabout60-70 mph then more vibration. Also getting the growling sound when clutch pedal released. Appears to only happen after warmup. One minute I think a pilot bushing, then I read about imput bearing, saw a video on clutches and was now thinking disc to light so springs making noise. Then Iím told drive angles. I guess Iíll have to pull the entire thing and examine. If I push clutch in the sound disappears. That tells me that would eliminate the pilot bushing and disc since the imput shaft is stationary. When pushed in, that would indicate the imput shaft bearing. I have two issues here. 1. Growling noise and 2. Vibration at higher spees(mph) revving in neutral does not give me vibration. Iím at a loss Thanks for letting me rant,