I e put a few hundred thousand miles on 5.4s over the years. Some of it towing trailers at least that big. 8 mpg is normal. Other than that, just let it wind up when needed.
I've horse whipped a few of em. Company trucks too small for what I was using them for. Never blew one up even over 100k mi. Have several friends with 5.4 fords. Older ones with high miles.
Swapping gears would be a waste of $ imo unless you got them cheap and did it yourself. Otherwise probably 1500-2000 to do it on a truck that is worth only double that?