Awesome, thanks guys. Seems 1 1/4" was what GM engineered that channel to be. My driver's side measures that consistently throughout the length of the channel. however, My passenger side is a 1/16" narrower in the rear and 1/8" narrower in the front of the channel. I have replaced my quarters with NOS metal.
I asked for that measurement because of my fitment issues with the front cap. I can get the doors to fenders aligned together, but I can't get the doors to quarters to sing in harmony. I have the tops of the doors adjusted further in to work with the fenders. By doing this, it pushes the door in past the quarter plane. Make sense? My only solution to this problem right now is to place a block of wood in the jamb at the top of the door and twist the door a tad to get it level with the quarter. I'm worried, though, by doing this it will kick that bottom of the door out past the quarter at the jamb.
-12* is cold enough to shrink a lot more than metal
I think the coldest it has ever gotten around here is 0. That was freaking cold. I could only imagine what -12 feels like. ouch!