1969 GTX. Does anyone know how much room is behind the door panel? Need to know how big (long) of a speaker can be put in without window interferience

I put these 6" Boston Acoustics Pro se 60's in my 70 Satellite. They are a 6" round. I don't remember how deep they are but I have no interference with the window mechanism. You could go a little lower than I did but I didn't want the speaker cover to hang off of the door panel. Its not always about the size, check into the Polk Audio component system speakers. I think they also have the capability to separate the speaker and the tweeder.

The tweeders installed on the A pillars...

You have about 2.75" of clearance for depth in the door..
When mounting in the doors, be sure to add the water shields as this will protect the speaker from water damage running down inside the door..

Just my $0.05...