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I'll disagree for the following reasons,
If the bell housing was far enough off center or out of plane with the back of the block it could create a binding on the input shaft that could lead to a vibration whether the clutch was in or out.





If that was the case just installing the transmission would have been a major issue.

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The motor may or may not be fine. Statements such as
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the crank finally broke at or near the flange and the carnage followed.

are pure speculation and will only make the OP feel worse than he already does



Oh, I'd be the first one to say that I hope I'm wrong but have been on the end of vibration issues like the OP described and if you look at the one picture of the crank flange it looks like the back of the flywheel isn't in plane with the back of the block.

The OP can test by placing a small prybar on the flywheel and see if the end of the crank is still where it is suppose to be.


Picked up a car with a 440 that had the wrong damper on it, shook like a washer machine with cinderblocks in it.