Great car that needs a good home. I have a soft spot for 56s.
You're correct in your assessment that identical door glass was used between '54-'56 Superior models, only placement changed. Measured '51-'53 Superior for you Kurt, see below. Without having quick '54-'56 access to compare measurements, I am certain that glass will not interchange with '51-'53. Top and bottom corner radii differ on '51-'53 while '54-'56 appear much closer. Exposed glass is first measurement; parenthesis contains best glass opening measurement 'guess' without removing glass.
When viewing comparison, keep in mind '51-'53 is a bit of an optical illusion as it cants forward more than the actual top to bottom taper. ('51-'53 and '54-'56 pics were not taken at exact angles.) Also, note croppings are not identical. '54-'56 is probably not narrower at base than '51-'53.
Who's your buddy???????