[patrickt]

A bad balancer is the only "easy" fix I can think of -- two seconds with the timing gun will tell you if it has slipped. I take it the engine seems to run right other than the really bad vibration? Was it dyno'ed? If it didn't vibrate on the dyno then that's a tip right there.

EDIT -- OK, I see it was dyno'ed. If it didn't vibrate on the dyno (and I can't imagine he'd let it go if it did) and it vibrates really bad just revving it at a stand-still, then you could take the tunnel out and disconnect the transmission, pull the clutch, and run it again with all that off and if it still vibrates you're going to have to pull the engine. That might at least let you spot some loose pressure plate/flywheel issues without having to pull the entire engine first.