Good answer above
The primary purpose of Xtra is to prevent fading. So, if you plan on getting a tonneau, then you don't need Xtra for its primary purpose. However, Xtra has secondary benefits though: Darker black, easier to clean, more chemical resistant.
Wheel wells look great with LINE-X. Xtra will help because it's easier to clean as mentioned above.
Not a Ford, sorry. Headers, yum!
