If you're just buying engine parts, the E-350 and E-450 parts are the same. Brakes, shocks and springs will likely be different between the two. Ford either has or can order whatever you need.
You might check your local area for a Fleet Pride store they usually carry parts for medium to heavy duty trucks and buses my last serpentine belt I got from them was a better belt than I could of gotten from NAPA and for less money.