The rack you are talking about is a calmini rack.
Safari Rack
They attach at the front overtop the door hinges. You can take these bolts out and the threaded part will stay there.
I just helped a friend put one of these on a tintop. It mounts differently on the rear because it clamps to the rain gutters on the tintop.
The only problem I would see with mounting the rear to the bumper and the front to the door hinges is
vibration.
The bumper is hard mounted to the frame and the door hinges/body is
not hard mounted to the frame. This may not cause any problems but that is the only problem that I can think of.
Good Luck with your build.
