the reason for the hinges being so close together, is you need them to be within the top and bottom body line, or they are going to hit on the gate before it closes.
I setup mine up twice, the first time I used only 1 hinge(bc my 2nd was slightly bent), but with only 1 hinge I got about 4 inches of sag...
So I got me a new hinge from the jy, and redid it, and now it sags maybe an inch at most, like leland said, you really only notice it when you first open or close the gate...
I think the easiest way to do it would be like leland showd, with the bolts and washers. And when you go to cut out the hole, I would take the hinge apart and just cut the hole big enough for the hinge to fit in from the inside. Then bolt the hinge to the gate, and reassemble the hinge. Set the gate up and mark where on the truck it will need to be drilled, for bolts and washers. Drill it and bolt it all up for good.
heres a pic with only the 1 hinge
and the redo with 2 hinges