Cage install??
Most chassis folks prefer to fit and weld their work themselves. This is done so that the joints are tight and the locations are precise.[Not saying you can't, just a comment

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As for the holes to the frame. It requires a considerable space to work with a TIG torch, even w/ the short cap on. IF you are going to be doing a frame -off type buildup of the car, then you can leave the body mounts loose, jack the body up and weld the tubes easily w/ them thru a hole saw sized opening.
I just completed the install of my 6 pt in my GN... NOT FUN!! I cut the floor w/ a cut off wheel and the holes are in the area of 3 X 6"
As for the plates, I used 1/8" plates to reinforce the frame where the tubes sit. I bent them to make an inclined piece that covered the top of the rail and extended down to the inner lower lip. I tacked the tubes first, pulled the assy up, 100% welded around the tubes and then welded to reinforcement plates to the frame..
My install is complicated as I have retained my door panels and back seat. [Even tho it's useless!!

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The rules say it must be welded to the frame. It does not have to have holes drilled so it goes thru, only on top of...
I also primed and painted the tubes after they were final fitted.. Makes it ALOT easier to touch them up vs totally painting them .
HTH,