Here's what other people had to say...
I've copied this from posts over the last year or so...

Logan426

I measured my 70 AAR and from the front edge of the doors, the screw center measurements are 17 1/2 and 20 1/2 and 15/16 down from the body line.

measurements taken from a 71 cuda from the front edge of the door to the front edge of the base. Driver side 16 3/4 back from the edge of the door to the base. On the passenger it is 16 1/8 from the front edge of the door back to the base.


05dakota

Passenger’s is same as driver's (talking about 70 challenger)


Classof70Chally

These numbers are from my car. (70 Challenger) The drivers side mirror has the leading screw 17.5" from the front edge of the door in a straight, level line. The passenger side leading screw is 16.0" from the front edge of it's door. All screws on both sides appear to be about 1.25 " below the first 'body break' line which is about one inch outboard of the window channel. (Are those mirrors original to the car?) As far as I know, yes. Would I bet my life on it?, no.

(Tav edit: measuring in a level line will produce flawed results when comparing two or more vehicles. Level is always level, but the position of the cars relative to level will differ due to leaf spring selection, tire & wheel height, suspension height setting, etc. The front edge of the door is curved. Measuring in a level line will result in the measurement coming from a different point on the curved door edge depending on suspension height etc.)


Hans

Both the left and right are located at the same distance (on my '73 anyway) (‘Cuda)


Alyerpal2

I know the front screw goes right behind the window channel slide. Just as the drivers side.