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.