It would look very weird, since the styling is completley different. There's also an issue with the length, other than the '69-'72 Longhorn (8.5'), the longest Fleetside is only 8'. Yo should be able to get an original 9' Stepside box, check with the guys that cut their 1 tons down to 1/2 ton wheelbases. The bedsides are also available new from Mar-K.
Another option, if you want a Fleetside look, might be to find a Studebaker bed. They're similar in general shape to the AD trucks, and might work with tweeking.
