I have a similar story. I had bought a clutch pressure plate and installed it in my car years before I ever took it down the road. I got my car running and had a bad vibration sitting still revving it up to about 2500-3000. I went trhough a process of elimination even fell for the "its the engine it isnt balanced " BS pulled the 340 installed a 318...still there, balanced the flywheel on a tire machine that heavy chunk of metal spun 0. it ended up being the pressure plate. I spent some money figuring that out but it was the same :"gee i bought that 5 years ago I cant take it back saying its a bad part" even though it was...:)