Gas mileage and 4x4 do not go together. I would have been thrilled to pull 10.9 from any of a number of 360-4, most of mine were in the 8-9 mpg range.

I never noticed much difference in which plug brand I ran, other the a few issues with AC plugs. Headers can help, if you keep your foot out of it, but I'm not sure the added noise, additional cost, and difficulty of installing and working around them is worth the extra little bit of fuel mileage involved. Headers do tend to add a little kick in the seat over manifolds, but one tends to get addicted to the kick and forget about the mileage.
Unless your doing a lot of driving and putting on a lot of miles, many gas savings parts barely pay for themselves in real life. Gene