I would really recommend you run the boat on the trailer over a certified scale. Our 25' C-Dory, with an 8.5' beam, on a galvanized trailer weighed 8,700 pounds. I think most R-27 owners are reporting around 11,000 pounds on the trailer. I would guess you are going to be on the far side of 12,000 pounds with a 31... a ton more than your 10,000 estimate.
To get an accurate weight, you will have to do two weighs: one with the truck and boat/trailer with the truck wheels separate from the trailer wheels. Then, disconnect, and have the tongue jack on a separate section from the trailer wheels - that will give you an accurate tongue weight, boat/trailer weight, as well as over-all weight.
Good luck with your trip. 1,400 miles in three days with a heavy load is pretty ambitious. No big deal for a big rig driver who does it everyday, but it is a different situation with a pickup pulling a heavy load. Your 3500 should be a good match-up, but that is a lot of weight and dynamics on the back. With a trailer with an 18,000 pound weight rating, I would imagine it has electric over hydraulic brakes. Some truck models require a weight distributing hitch for weights over a specified amount, as well. Tow/haul and manual shifting with the Allison transmission is something you will want to get familiar with.
Jim B.