Hey Tomas,
I am currently dealing with the same things. The end tab where you have marked with the 1" hole uses a Phillips sheet metal screw to connect to the top tab of the rear quarter on each side. If that tab is too short to make a connection, weld a small extender piece onto it so there is a place to mount that screw.
My 72 did not have those mounting tabs in that position. My rear pan was only connected with the four tabs on each edge of the pan, two per side, the two sheet metal screws and the center brace that connects to the tail panel cross member. I am modifying the the cross member and making a small mount on each side to make a connection near the bumper guard hole to straighten out the pan. It will have to mount on the bottom of the cross member to miss the bumper mounts. Just a suggestion you may consider doing.
Slotts