Okay, before I get flames for suggesting this, I'M NOT GOING TO DO IT! It's just that I had plenty of time to think about it a day ago.
Anyway, I've got a '97 faux F250 with the 5.4/automatic. When I say "faux", I will say that it's got the 7-bolt wheels and does not have the full-floating axle. So in reality, it's only a heavy (or heavier, LOL) half ton, it's just badged an F250. Mileage is high, but I don't drive it in the winter so it's in pretty good shape.
Anyway, I just did about 12 hours of towing with it. I towed a 4500 lb. vehicle with it and going up the hills it was...well....somewhat anemic. The power just wasn't there.
So, with plenty of time to think about getting more power, how hard would it be to drop in a 5.4 3-valve? A few of our trucks at work have them and I know the power is there, are at least more than what my truck has.
Once again, I wouldn't do it to my truck. I'd buy a newer truck before I'd get crazy and swap a 3-valve into mine. But, how easily could it be done?

 

I have a 2002 F150 with the 5.4 and I have pulled a 12,000 trailer for short distances. I have pulled a 9000 trailer all day long.
Computer match up would be a concern on any motor change.

 

it is doable, have seen a write up somewhere. You just need lots of stuff for it.
A better, cheaper and more viable option for a little more power is swapping the heads for a newer set of PI heads

 

Personally I would look for a 5.4 from a 99 - 03 if you were serious about swapping motors. In 99 the 5.4 recieved a significant power and torque bump. I have an 02 with 182k miles on it with 3.55 gears and it pulls my 6000 lb 5th wheel pretty well. I recently replaced intake manifold gaskets and that helped restore some life into it. Also have intake and cat back exhaust which help too.