And unfortunately, the BOM may not tell the tale of what might have been swapped in over the years.
But if you know for a fact that the brakes, and more importantly the knuckles, are from a '76/'77 Bronco, and not a full-size truck or Bronco, then at least you know which kit you need to buy. They're completely different and one does not fit the other, so it's good to know up front.
But what exactly do you mean about the "six-piston kit" fitting the stock rotors? Are you talking about installing Wilwood calipers without their rotors? The kits I'm familiar with all include matching Wilwood rotors. Unless they sell the calipers and brackets by themselves and you're trying to make them work with the rest of the factory stuff.
In that case, what Todd said.
Paul