Slow down guys. 2 5/8" is the stock OD for a monojet, which may have been the original carb on your truck. Somebody like George or Bill can tell you when the "B" ended and the Monojet began, but they both bolt to the same manifold opening. Why would the factory have to "adapt" an air cleaner, anyway. My guess would be that you have a combination of parts that don't all match. I could be wrong though, just ask anybody!
Best wishes,
Les
