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I guess you have looked it over and the wiring is different? The reason I ask is I looked up a 76 LTD with a 351(I guess it was a w it could have been an m) and then looked up a 86 f150 with a six, and a couple of the alternators where the same for both.
I say a couple, because your year had at least two different kinds. Your 86 wiring may be for an internally regulated alternator.
What happened to the original six alternator? I just looked up an 86 F150 with a 351w 4bbl, and it has the same selection of alternators the 86 six cylinder has.