New bumper install problem

[gmcdude]

I have this 72 GMC Truck, when I got it there where no bumbers or other items installed yet due to the pervious owner quit the rebuild. so thats where I came in. I ordered a new bumper and new brackets, I'm having trouble getting the brackets , I've bolted the bumper to the frame and as of right now the driver's left side seems to be real close to the frender compared to the other side. as for the brackets does the oblong hole go to the bumper or frame, and does it bolt to the frame in the front hole or back hole? Help Please