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Did you take the torque convertor out with the trans, and then fit in reverse order?
In other words, the convertor and transmission should stay as one while removing or fitting.
What was wrong with the original transmission?
Yes, that's possible, especially if the engine has a higher output than stock.
Has the new transmission been rebuilt, or is it second hand from a wrecker?
Was the torque convertor front seal replaced prior to fitting the trans?