Allen, I've not been made aware of any supplier for running boards or valance/filler panels for any AD truck other than the 1/2-tons. The only two sources I am aware of are either finding used ones or fabricating a set from several sets of half-ton boards (like you are considering).
As for "long" versus "short" beds on AD pickups, the Chevrolet Truck Data Book tells us that each rated size pickup only came in one bed size: the 3104 (1/2-tons) have 78-in beds; the 3604 (3/4-tons) have 87-in beds; and the 3804 (1-tons) have 108-in beds. Calling them "long" or "short" is not actually meaningful nomenclature.
Good luck finding a set -- they are out there but getting harder to find. And they will probably be $$$ when you do.
Regards,
John
