Understand that this is third-hand information, so take it for what its worth... but, I have a friend with a Ford truck of this era, and he ran into the same shop-manual instruction to discard and replace the flange bolts. He questioned a mechanic friend about it, and was told that the factory-supplied bolts come with a thread-lock/sealer applied. The coating has a one-time use, but the bolts themselves, if they're not physically damaged, should be fine. So long as they're thoroughly cleaned and new thread-lock compound properly applied, they're OK to re-use. In a commercial shop setting, its often not worth the mechanic's time to do that, so they typically use new, pre-coated bolts when available. But for an owner-performed repair, the calculus is probably different.
Again, its third-hand information which I have no way to independently verify, and standard disclaimers apply. But it sounded like a reasonable explanation to me.
