I've noted that towing capacities are higher for a 5vr vs. a tow behind too.
I'd keep the fronts under 80 psi and 50 is a good start. Once you get the trailer on the truck you can experiment a bit.
GVWR does not assume whether brakes are present or not. It is simply a measurement of how much the truck can haul, loaded. Basically the weight of the empty truck, add fuel, add passengers, add cargo, etc...
Here's a great site with some really friendly folks that can probably shed a bit more light on your truck and towing: