[69-72]

The rocker boxes keep the body from sagging when the top is off.
If I were to install rocker boxes I would have the top on and the door gaps set. You will need to install the rear pieces of the rb that bolt on first and then weld the rest in place. If you were to weld the main rb on first and then moved to the rear portions it's likely the bolts wouldn't line up since there are just so many bolts and variables.

You may need to trim under the rocker panel for it to fit. I am not sure what you mean by drilling holes in the front of the box, originally it was just a couple weld beads on the front lip. If these are reproduction pieces don't be surprised if things don't line up perfectly.