Nice build.
FYI, the real reason frame rails in pickup trucks are not fully boxed is actually a really good one - when you put a couple thousand pounds in tongue weight and cargo on the truck, a boxed frame rail is gonna snap like a toothpick. Simple C channel frame rail will bend with the weight.
It has nothing to do with strength of frame - my 8000 pound F350 can handle itself just fine, and its C channel all the way. Boxed rails are about ride comfort, as a stiffer frame absorbs vibrations better and keeps them out of the cabin, and prevents torsion/flex in high torque applications.