A 4 door and post car use the same chassis/roof. When the car is built as a post an interior 'plug' is used where the back window is. To do the job correctly, a post car donor needs to be used for the plug/jamb, post/glass and doors.

It doesn't make sense to me to do all that work since a donor post car needs to be located anyway.

Then there's the issue of getting someone to sell you a car knowing you're going to cut up a post car to build a post car...