Just to possibly offer another idea....
I used to be using a single shear spindle hinge, but recently replaced it with a 4xInnovations hinge. My bumper wing design is different from yours but I also had part of the hinge sticking out past the back edge of the bumper and supported it with gussets. As for the bolt... I suggest putting the nut on the bottom. That way if the nut somehow comes loose the bolt won't fall out and let the swing arm fall off. If the nut is on the bottom the bolt stays in place.
with the nut on the bottom you could still drill the hole in the bumper so the hinge sits closer to the body. I welded the nut to the inside of the bumper so it's a captured nut and I just need to put a wrench on the bolt to secure the swing arm instead of trying to hold a nut and a bolt. Even if the hole you drill for the bolt is close to the back edge of the bumper you could shave down the nut to fit and weld it as a captured nut.
Ignore my very marginal welds. They're strong, just not pretty.
Untitled by
Adam Tolman, on Flickr
Untitled by
Adam Tolman, on Flickr
Untitled by
Adam Tolman, on Flickr