OK, here's the scoop on early Ford axles.
1. The kingpin boss thickness and hole ID are the same for passenger cars 28-48 and pickups (1/2 ton) from 1928-47.
2. All the spindles from passenger cars 28-48 and pickups (1/2 ton) will fit all the axles when using the appropriate kingpin set.
3. The 28-36 spindles are not often used on rods because they do not accept the early hydraulic brakes without adapters, and because the aftermarket disc brake kits are made for 37-48 spindles.
4. The 37-41 spindles are known as "round" spindles and they 42-48 as "square" spindles. This is from the shape of the brake backing plate mounting flange on the spindle.
5. The axle bolt or spring perch boss on the axles are 2 different thicknesses. This is the dimension mentioned in the catalogs which is critical for buying batwings and other suspension components.
Of course, there are overall length differences, spring perch boss width differences and "drop" differences over these years as well.
Hope this helps.