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You probably know this but 360's are externally balanced,heavier dampner and weights on the convertor.Since yours does'nt have the weights that is probably it.And check and see if your u-joints are centered in the yoke.

If he already bought the B&M flex plate for a 360, it is made to be run with a non-weighted convertor, which he has. He should be all set there. Cheap aftermarket convertors can have a balance problem - hence rotate it 180 and see if that helps. If he put the u-joint "C" clips in properly, they should be centered. Good to check though.

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