This is purely a guess but I'm assuming that the guards screw into some supporting rib structure. With that in mind I would think that the '85 not being a wide body, that the hinge and other mounting hardware would be positioned incorrectly based on the narrower width of the trailer.
Jack
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Jack Canavera
STL Mo. AIR #56 S/OS#15
'04 Classic 30' S.O.,'03 GMC Savana 2500
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