proudcelt,
I agree with the FIPK. Colossus is right about the hot air, but even if it's hot air, it's getting a crap load more than what it was. I had a '99 Expy, put on a FIPK and the engine was a different engine. The thing wasn't gasping for air anymore.

Back to the question, the 5.4L would handle a 5k boat, but the problem is, in my opinion, the suspension. I towed a 2250l. boat with mine and it was a stinkin yoyo in the back. Definitely get some sort of air bags or something that will help stiffin the ride when hauling the boat...