All window air conditioners I have seen had a specific mounting point and there was usually a metal strip or something there for the window weather seal to sit on. I have also seen others that had a seperate mounting bracket and the air conditioner slid into it.
I think you will have no problem determining how far it should intrude into the room. I would just make sure it is slanted down on the outside for water to drain and provide plenty of bracing to hold up the rear of the unit. There will be a lot of bouncing around going on back there when you travel down the road!
Barney