I have my old Ford hotrod truck from my 20's still and if my memory is correct the trucks used 31 splined axles and the cars used 28 splined axles. I would research or pull an axle before I baught any parts for your 9"; it is probably a 28 spline axle though.
The ford 9" used to be the choice of hotrodders because it is the toughest differential ford built ( but the carriers have different castings and are important also). I have always gotten consistent double tire burnouts with my truck wich has a modified 460 in it, I have a limited-slip differential also. A spool is mostly used by serious drag-racers and 4x4 drivers because it is just a solid gear holder that locks the axles together, on the street it will chew the heck out of your tires in no time flat.
Hope this helps and nice car BTW.