67 Barracuda Formula S 451 - clutch shatter

Finally we have solved the problem :)

Further things done were change of the 8.75 gears to 3.23 instead of I think it was 2.76 - the shatter was a bit less, but still there.
Then we have took of the clutch again and changed the flywheel for a new one and went back to the Centerforce clutch. Problem still there.

The last thing we did not change was the driver side engine mount, as there are very limited options on the market for big block A-body engine mounts, and we did not want to go for a Poly engine mount on the first go. And we thought the torque strap would eliminate all engine movement on the driver side - BUT before we went to the next steps, we have noticed when pushing the clutch pedal, that the engine moved a tiny bit to the front - that movement is not blocked by the torque strap, it only blocks the side-movement, but not back/forth.
So just to see if it could be related to the engine mounts, we have machined some aluminium blocks as solid engine mounts as a test.
And voilá - the problem has completely disappeared! No trace of chatter anymore.

The lesson learned, is that with a weak old driver side engine mount (even when having a torque strap), while engaging the clutch, the engine torque and take-off accelleration of the car pushes the engine to the rear on the driver side and slightly disengages the clutch again, the engine moves to the front, the clutch engages slightly and the loop goes on and on, resulting in the massive chatter.....

As no other options are available we have bought a set of Poly-Loc engine mounts from PST and installed them - still perfect - and with even less vibration than expected with poly mounts inside the car.