Well I would guess its a 31 spline simply because it is a truck axle. Although I am not sure about the F100s. Fastest way you can find out is remove the wheel, brake drum, unbolt the axle (4bolts) and pull it out. The 28 spline axles are tapered and the 31s are not.
As for that engine all you can do is look at the rear of the crankshaft for determining the stroke, unless, you want to tear it down n look at the crank casting number.
Always pull the axle if you are not sure, it is pretty easy to do for the 9 inch. I do not think I would run 28 spline with a spool anyways.
360s always become 390s when they want to sell the truck. I knew nothing about FEs (except the 351 air cleaner was wrong) and my 1971 4x4 which never should have had a 390, since it only came from the factory with a 360, actually was a 390 from a 1965 Galaxie 500.
Easiest way is pull a plug and measure the stroke, 4" = 390. About 3.5 = 360?