So before starting to drill into parts that have taken so long to produce, I decided ot draw the parts to scale on a piece of graph paper. I felt that it would be very difficult to just try and rivet the -4 bolt nutplates to the rectangular tube mount itself so I decided to make a backing plate to go inside the tube and mount all the nutplates on the backing plates including the #8 nutplates that would hold the backing plate to the tubing.
I was able to work out all the dimensions and fabricated the backing plate. It will take a couple processes to get it complete. I drilled the backing plate in line with the tubing with the backing plate on top. Then I deburrd, countersunk the rivet holes and the screw holes. Next I will mount the backup plate into the tube and place the mounting angle on top to match drill the angle / tubing / and backup plate for the #4 hold down bolts.