The 56 F-100 does not have the same gears and interchangeability of the 57 up rear ends ...
so the original 56 F-100 rear end is not as user friendly. I would select a 9 inch Ford rear end ... from this donor list ... with the width you want ...
http://carnut.com/specs/rear.html
A rear end from a Ford passenger car would have the same wheel bolt pattern as your front end has now ( with the 5 on 4.5 pattern ) ... and use a ratio that is very close to the ratio in the Formula rear end. Do you know the ratio in the Formula rear end ??