If you think the 4.6 wil get good mileage in a truck you will be sadly mistaken. We have several, about 10, F150's. Some with the 4.6, they get maybe 13 mpg and are fairly gutless. My work truck has the 5.4 and it get's 18 or so, plus or minus .5 mpg depending on if I have a tail or head wind.

Good luck on your project but I suspect you willbe disappointed in the results. Your truck has the aerodynamics of a brick wall and the 273 will have to work harder to overcome that. Simple fact there. It takes so much power to overcome it. The 360 will make that power at a lower rpm than the 273, take so much gas to make a level of HP that regardless you will still be burning that gas, or more, to push your truck around.