What kind of vehicle are you going to put it in and what is the application?
A neat way to narrow a 9 inch: IF you have the 28 spline 9 inch rear end, you can graft the axle tubes from an 8 inch out of a Granada or Maverick, or one of a myraid of Fords produced with the 8 inch rear end, to the center housing of your 9 inch. The 8 inch also incorporates the 28 spline axle and they will fit perfectly.
The 28 spline is a knurled spline while the 31 spline is a machined spline.
OR for that matter, save yourself a lot of cutting etc, and just use the 8 inch rearend. The biggest limitation of an 8 inch is gear selection. You cant get as deep of gears for an 8 as you can a 9, but I suspect, unless you need deeper than 4.63 gears, the 8 inch will do the trick. While a lot lighter in construction, the 8 inch is a very sturdy rear, and will handle most anything on the street up to around 450-500 HP.