Will my engine fit please help?

i just bought a 2011 ford fusion with the 2.5L I4 engine... i know they have a model with a 3.5L V6 engine but i could not afford it.... i was wondering if the engines are interchangeable and if the v6 would fit in my car.... also if anyone knows a website where i could buy engines because i know ford will charge me an arm and a leg for it.. thanks!