The f1 box has splines not a key way. You can swap the shaft from a 37 to 48 car or up to 47 truck depending on the length. The worm is the same. Common way is cut the shaft and weld the piece on with the key way
Napa has woodruff keys . The ford one has a flat spot on the back side to clear the horn wire or horn rod on 39 and older
The nut is a half nut size I think is 5/8 fine thread off the top of my head. Get a full size nut and split it in half. Cave man style is with a hacksaw or turn it down in a lathe