Couple thoughts. I would check your state law, and see what restrictions are on the books R/E load length past the bed of the truck. A support that attaches to the hitch works well. I use one for a 16 foot kayak. Bigger is always better, as long as you can load and unload it. Be sure to check the weight capacity plate on the hull. When you figure your load, its just not the people, but gear, motor, fuel, everything total, with a reasonable safety margin. There are caddy type devices that allow you to wheel the boat to the water, if needed. Again, the one I use on my 2 seater kayak works well. Unless you are going to be on really choppy water, a flat bottom Jon is a lot more stable then a deep v bottom, it just rides rougher in the waves. Most people consider a welded hull stronger then riveted, but usually more expensive. For what its worth, I have a 1987 Sea Nymph 20 foot tin boat I bought new, and use it quite a bit on lake Ontario, in some really rough water. It has a riveted hull, and still floats like a beer can (with dents), no leaks.
Larry