why a Ford 9"?

The link posted earlier states the 8 3/4 uses 29 spline axles. Ford axles were 28 or 31 spline and the 31 spline axles tend to be later model as opposed to any intentional use as heavy duty. Still, more splines = better and I'm sure the Ford engineers knew this. At any rate, change the side gears and add custom axles and you can pretty much get whatever you want, but I'm thinking along OEM lines here.

One application specific item with an A body is gas tank clearance. I'm pretty sure I read somewhere that the 9" Ford in an A body creates some hassles here as does the Dana 60. 8 3/4, 8.25 and 7.25 fit no problems.