Hmm, they didn't make 8 foot stepside boxes after 79, maybe you could take one of those and put the 80-87 'glass fenders on it. I don't know about the LWB boxes, but the 80-87 SWB Flareside boxes are wider and taller than the earlier SWB flareside boxes, maybe you could graft in some sheetmetal on a later box.
thats what i figured i'd have to do. i think it will look good as long as i get the right fenders fot it. the bed i have on it now dosent have much floor left to it. the sides are good. i was thinking if i cant find what i want for a bed i'll put diamond plate down for the floor.