I had something simular happen to me bout 3 ago. Paid $6000 for an apparent Hemi 0range 318 Duster that had the engine swapped to a 340 727 trans combo with an 8 3/4 rear. Official paper work concuring what the seller said etc.. I didnt know enough about I.D'ing engines etc... at the time needless to say the engine was never swapped just rebuilt in the 80's and it had a 7 1/4 rearend. Tried to go after the seller but would have cost more $$$ then it was worth plus court time.
Car is pictured below.