The only 351w in a "truck" ive ever seen get better than 20mpg was a built 351w in a little bronco that has a fiberglass body. But like the point I made earlier, it doesn't make sense to get a truck that is way more than what you need.

 

if your after 20 mpg in your current f150 351w gas truck,you should consider lowering it,skinny tires,gearing to 3.08 and a cummins 4bt swap.i bet you could nail a true 20 mpg or better lifetime average.not just a possible peak here and there running her empty out on the flat interstate,downhill with a tail wind.
try to keep realistic with your plans and goals,otherwise you'll end up very dissatisfied with the sub par outcome of them.
plan on 10 mpg with a 351w truck and you'll be satisfied with it and happy when she does a bit better.

 

i know i will never achieve 20 mpg, not what im after, i know theres a guy that gets that, idk how but he has gotten quite often. Im more after awesomeness, those power mods are only going to get done and avoid my parents nagging me about it is if i can plead gas milage. I plan on lifting it and once that happens down goes mpgs but i want the power first so i wont suffer terribly