I would skip the pre-made felt gasket entirely. I have changed the diff fluid in my Ex twice and used Permatex's gear oil RTV both times. They make an RTV specific for the differential (Permatex part 81182). A 3 oz. tube is about $9. I have had good luck with it; no leaks.
I bought the Lube Locker reusable gaskets for my axles when I bought the new housings with 4.88 gears and added the '08 SD aluminum cover.
They cost more than a tube of RTV but they can be used over and over so maybe in the end it's a push on the cost. No leaks so far with lots of heavy towing. LubeLocker - LubeLocker Ford/Sterling 10.25"/10.5" and for the Dana 50 up front LubeLocker - LubeLocker Dana 60
This forum is owned and operated by Internet Brands, Inc., a Delaware corporation. It is not authorized or endorsed by the Ford Motor Company and is not affiliated with the Ford Motor Company or its related companies in any way. Ford® is a registered trademark of the Ford Motor Company.