Here's an old school way to find out your gear ratio. I have done this a few times myself back in the day:
http://www.youtube.com/watch?v=RNgjS5mHI0E&feature=youtube_gdata_player
I am running 265/70s, with no tbar cranking or plastic trimming. I have a buddy running 285s but he has a leveling kit on the front of his 2001. I'd wager that you could run 285s if you crank the torsion bars all the way, and you might have to trim a little plastic to prevent rubbing at full...
I get very nervous when I get close to 1/4 tank. My lift pump moves a lot of fuel, and I have the stock pickup. I should have gone with the billet pickup a long time ago.
I installed the blocker plate at the firewall. It is a reach, but not too bad if you have a jointed socket.
I unplugged the EGR, but you can just leave it alone once the blocker plate is in place. As I mentioned, my truck did not throw any codes when I blocked and unplugged.
Is your son's boat on a 5th wheel trailer?
That must be some boat.
There is no way that I would attempt to tow anything on a homemade hitch, especially something with the weight requiring a 5th wheel trailer.