From everything that I've read the real problem is the GVWR number on the inside of the door post. You could change all the parts for your truck to be an F650, but if that tag doesn't change, and it can't as I understand it, then your towing capacity is frozen at the sticker's weight rating.

The other thing that I've gathered from reading the posts on this site is that the axles are more than capable of handling the highest weight ratings put forth by aforementioned sticker.

Last thing is that the axles are the same for F250 and F350 SRW vehicles. There is a difference in the DRW vehicles (Dana 80 if memory serves). You're looking at major changes and expenses to convert from SRW to DRW, if that's what you're asking.

Let us know what weight you're trying to tow and what type of trailer, i.e. goose neck, fifth wheel or traditional. You are might be capable as it stands, but might want to gear down from a 3.73 to a 4.30 to "feel" a little strong for towing. Or maybe you're looking for more stability.

Be as specific as you can and you'll get some awesome specific answers here. This site is awesome for that type of information.