Hmmmm decisions decisions..
I'd say go the shortbed route because it would be the least amount of work and cheaper than buying a shortbed truck. Also using the 2003 frame may be a pain to get the bed to sit low enough on the frame or the cab high enough to line up with the bed without it looking odd. If you want another longhorn based truck, I'd start with a longhorn. Then you will have a shorty and another longhorn!
Are you inspired by the Big bird build Bluebullet did
?
If you find an extra shortbed, I need one for my 72 k10 swb truck that is in pieces!