I just installed my 6637 air filter and with the suggestion from a guy at work I purchased a 3 inch rubber coupler for pvc pipe at our local hardware store and it also comes with 2 hose clamps it fits perfect inside the hose and filter the outside diameter is 4 inches and the rubber is thick enough it bolted up perfect and works great and it only cost me $6.00.
Hope this will help if you are installing a 6637 on your truck.