installing hitch in 2003 GMC 2500HD 8 foot box(updated)

Thanks for the replies, I will have to check it out, as the regular 2500 shows drilling through both frames and using 4" bolts, but this kit and instructions for mine show only the 1 1/2" bolts and not drilling through both sides of frame, only one side which leads me to believe it is an open frame in both places, the only other thing because of those bolts having serations that is why they have to go through from inside frame, and not through brackets first I gather.