Hi Team
Is there a factory location/dimension to install the passenger door external mirror

Try the same position on the passenger door i.e. same dimension from the front door edge and the window.

On a 70 Body mounting a mirror on the passenger door matching the driver's factory side position results in a very poor viewing angle for the driver.
So one has (2) choices, either mount the mirror to match the driver's for best visual appearance or modify/mount it in a different, slightly more rearward position.

Just my $0.02...

The right mirror on my 70 Charger is practically worthless for viewing. I think farther back may be worse. I have to lean back to even get any image. It’s the vent wing post in the way for me and just a bad angle. After driving a modern car or truck with good mirrors, we get spoiled.

On my '68 RR I added a passenger mirror positioned exactly like the drivers side and had a good view from it...should be the same as a '70 GTX.The view depends a lot on the seat position, I usually have the seat all the way back.

Was your driver's side mirror remote control?
On my 70 RR the driver's mirror was OE and remote control...
I mounted the fixed passenger side mirror in the same position and
it is useless for viewing this is with the bench seat all the way back..
But is balanced mounted identically compared to the driver's side..

Just my $0.02...

Right mirror placement has always appeared symmetrical to the left to me.
It's been a fairly rare occasion for me to trust them completely as precision rear-view locators, so to speak, since these cars were new. They're pretty small.

I bought one to match the remote driver side from Tony d and he said to just lay it out like the driver side to have it like the factory did it. I have yet to get around to it, so I can’t say how that would do. I had a 68 charger about 25 years ago and I don’t recall the pass mirror being useless. I have to have my seat all the way back fwiw

Well, like all external mirrors of that era, I wouldn't say "useless"...more like "limited".

I added right side mirrors to two cars and the main point I found was to be sure the wing window still opens! After that I found the position was functional as I really needed the mirrors for getting in and out of my garage.

Actually one has options when installing a right-side mirror...
1. Mount the mirror in the same position as the driver-side remote OE mirror
2. Mount the mirror in a better viewing position but then typically there will be a mismatch compared to the position of the remote OE mirror

Just my $0.02...

1966-67 the mirrors were mounted forward of the 1968-70, you looked through the wing glass to view the mirror. This only slightly blocked some of the glass, but the right side mirror is very useful to the driver, you can see down the side of the car when normally seated, no leaning.

1968-70 they moved the mirrors back behind the wing so you look through the door glass to view the mirror, this is too far back so the right mirror is useless when driving and I have to lean over to the passenger side to look down the side of the car when parking.

I'm not tall and have a small back side so the seat is in a forward position.