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.
Evan MacDonald