As these cars age, it is common to find them with replacement mirrors from other models or years. It is also common for the threaded inserts to be missing entirely. Also, they do not all attach the same, some use nuts on the inside of the doors and some have the holes farther apart. Here is how the factory did it on 1970 E-Bodies.

In the photo below you will see two threaded inserts and a correct mirror screw. The threaded insert on the bottom is an original, and the one on top is a generic replacement from NAPA. The thread size is 10 - 24 UNC. Threaded inserts are also commonly referred to as poly nuts or rivnuts as well.

In this case, I am adding a passenger side mirror to my T/A, but this same information applies if you just need to install or replace missing inserts. Take careful measurements to get the holes in the right place then drill with a 1/4" drill bit. The next photo shows the two new holes.

Here is a photo of the Thread Setter tool with the first insert attached and ready to install. Follow the tools instructions as they may vary depending on the brand you use. At this point, just slip the insert into the hole, hold it firmly and squarely to the door surface and then squeeze the handles until they stop. Remove the tool from the insert and get it ready to go for the next one.

Here is a photo showing the second insert being installed, note that the first insert is already in place.

At this point, you are ready to install the mirror with the correct screws and you are done.

The thread setter kit is available through Napa, part 770-2875. You can buy the inserts you need and the tool separately, but for not much more you can get the whole kit. Dial 1-800-LET-NAPA for a store near you.