[dsraven]

if using the s10 frame and it is too short, look behind the frame drop around where your foot would sit if there were a cab on it. there is a spot where the front meets the back and guys cut the welds and slip one side into or out of the other. maybe you can just "remake" the rear half with rectangular tubing so it is long enough for the longbox? keep your eye out for a longer s10 that you can get the rear part or the whole frame from? meanwhile you can still fab up the mounts etc and be ready.
how about a wrecker that would possibly sell the rear section of the frame, dissassembled properly, without the diff, springs, fuel tank etc etc. those parts are the money for the wrecker anyway. they would likely just be scrapping the frames in the crusher when the money parts are sold. might be worth a phone call at least so you know.