I have a 72 Mach1 with a fresh 429 in it. I am having a vibration problem and cannot figure where to go next to solve the issue. I know it is not the rear or driveshaft since the vibrations happens when I am in nuetral and parked. I have already replaced the the harmonic balancer with no change. It vibrates at different rpms standing still or moving. I am wondering if it might be the torque converter or something internal. Anyone have any ideas or some type of test to try and narrow it down to one problem? Thanks a million in advance!!!!!  

Lots of veiws but no responses. Does anyone have a clue cause I sure don't?  

do you have a 460 flywheel on the motor?  

I would look at the flexplate  

I had a similar problem, and found to be in the tranny, if you have the facilities, remove the tranny and run the motor alone, if you have a problem with th t/c, it should be leaking by now.  

I have a 65 Falcon that vibrated pretty bad and it was a "brand new" Flexplate I bought off the net. I messed with that for a while because I didnt supect it being new. I got an old plate from a friend off a 289 and put it in my 302 "no more problems"
D In Detroit  

Yep Flexplate
I had the same issue with a 302 and changed the flexplate no more vibration