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Towing generally has more to do with options on the engine rather than the engine itself. Increased cooling is a main point of consideration when evaluating the differences. Otherwise, the tranny and rear end play a critical role.
You can build a 2.5 4 cylinder ford to be more powerful than the 460. My brother in law used to do it all the time in his race car. However, if you spent as much money on the build as he did, you would have a 460 with at least 800 horsepower. With a 400 the number will be slightly less. The 400 is a good motor - and has a really long stroke which is perfect for towing. If the 400 is already in the truck I would build it up instead of the headaches of swapping out to a 460. I do think you will be very happy with a 400 with a good intake, carb, cam, and headers.