GA_FORD - "...is it possible to shorten the frame of a longbed 2003 F250 6.0 diesel and put a short bed..."

But why? You bought a nice heavy built truck & now you are wanting to take some of the cargo room away? Going from around 80 cu/ft down to around 65 cu/ft.

 

Look here. Truck was originally a reg. cab long bed, now a short bed. Only thing I don't like is that they should have "fish mouth" cut the frame, instead of making just a vertical cut.
http://www.earnhartbuilt.com