roadmaster trailer hitch

I am picking up a used class 4 hitch for a 99 GMC truck I wish to attach this to my box frame on the buick. How does the genuine frame hitch made for the car attach to the box frame on
the buick, do holes have to be drilled? The box fram has little access from outside and I dont see how a nut could be placed inside the box frame. I am good at welding fabrication etc... so I can make this work eventually. Buick frame is 41 inches and GMC hitch is 39 inches.

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