I'm not 100% if there's anything special with Ford truck oil pans but with most cars and trucks when you drain the oil and take the pan off, sometimes some of the gasket actually sticks to either the pan itself or the block. If this is the case you're going to have to remove every bit of it. Using a paint scraper works good cause it's flat and wont leave small scratches from say a screwdriver, as an example. Once its all off, just briefly go over it with some sandpaper to make sure it's clean and then wipe it off. All of this helps to create a smooth and true seal.
Again, this is what I've done in the past to some of my cars and I hope it applies to you here.