You wont have to pull the heater on the passenger side, just the glove box Yes, there are backing plates behind the sheet metal.
I used a spring loaded center punch from the inside, then just drilled through the dimple from the outside. There are 4 holes in the support bracket on the inside. Make sure you use the correct ones.
Ive attached pictures from Denny Graham's web site to show passenger side.
Drivers side similar
Not sure on the level of restoration your accomplishing, my 50 has a fiberglass/body filler repair on west coast mirror holes in the door. Its held up for 39years. Weld repair would be better.
