The camshaft dictates the firing order. If your engine runs on the 154... firing order, either the camshaft has been changed out or the whole engine is from a vehicle that did not have a H.O. engine.
Not every 5.0L engine was H.O. From '82 through '86, only the Capri/Mustang 5.0L was H.O. The 5.0L engine in all of the rest Ford passengers cars in those model years was S.O.
S.O. = 15426378
H.O. = 13726548
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