Unless you are positive there are pits in the bore, I would not lap it. If it just light, to moderate, surface rust, I would give it a good soak with some sort of rust cutter. I would suggest AreoKroil, or Balisol, if it’s light, and a dose of Evaporust if it more serious. None of these will damage the bore, but over zealous lapping sure will. Most of us would give our eye teeth to have a barrel with old Bills name on it, so please be careful with it. When I got started a half lifetime ago Bill Large barrels were the holy grail in muzzleloading barrels.
Hungry Horse