ummm i dont know if im wrong but doesnt the transmission give power to the wheels and since the speed6 is awd and speed3 is fwd wouldnt that make them different
slightly OT but same ballpark... car & driver mag took a mazda5 and tried to install the MS3 powertrain. read here
http://www.caranddriver.com/reviews/hot_lists/high_performance/mazda_mania/return_of_the_boss_wagon_mazdaspeed_5_project_car
but bottom line is that, even w/ the help of a comm college auto class and an ex-crysler engineer it turned out to be a pain in the gluteus maximus. i can't even imagine what swapping out differentials etc must be like...