I know that for stock axles:
The 3/4 Ton axles are built to take larger king pins and heavier
You could not put 3/4 ton spindles on 1/2 Axles safely, even if by
machining you could make it fit.
Also the locking pins wouldn't work.
So I wouldn't try it. It might be unsafe.
I don't have the exact measurements for the king pin diameters, but
as I remember for the 1/2 tons it is about .86 inches. The 3/4 Tons
have about a 1.10 in diameter.
You might simply consider using a stock 3/4 Ton front axle. It should
fit the 1/2 ton truck. The connecting rod to the steering column
should be the same for 3/4 and 1/2 tons
You can take the 3/4 ton axles to one of those heavy truck repair
outfits and they will install new king pins. They are probably too
difficult to install without special equipment.
check out www.chevsofthe40s.com