The Youngstown doors on the Athearn model are properly sized. I suggest using that model (or extract the doors from it if you require them for something other than a 50' PS-1).
Thanks for bringing that up. I've been wanting to find my Athearn cars to see why they didn't come to mind.
Here's a few things to know when looking at the Athearn door.
First, I don't think it matches the doors used on 40' box cars. The 50' doors have a thicker boarder on the sides than the 40' cars. However, you may be able to sand the sides down to something close to 8'+ (the door is 9' stock) and then just maybe have an out of place tack board.
Second, it won't fit the MTL car without modification to it or the car (or both).
Not a bad place to start looking depending on what you're trying to do.
Jason