doughnut spare tire width

How wide would I need to leave my spare tire well in the trunk , to fit a "doughnut"?

Im doing the mustang tank deal on my 51 plym, and if I slid the tank all the way to the left, I could possibly save 4.25" of the original spare tire well. Would that be enough room or should I just center the tank and forget it?

jeff

 

They come in different sizes. The last one I had was 5" wide........

 

anyone know if they come as narrow as 4.25?