Woody, Classic Parts #53-312 relay is for 12V and is the one that I would use. If you look at it, the terminals are marked H, B, and S. As to your harness not having the wires I'd call the company who made it and ask them. I can scan a picture of my diagram and it would show you what to add if you want. I'm guessing that you have a wire from the horn button coming out at the end of the steering column.
As Bill says, the relay that Classic shows for '57 up is also a 3 terminal relay but uses push on wire connectors.
I just looked at my Chevrolet Truck Shop Manual 1948-1951 and it doesn't show a horn relay. It shows the wire from the horn button going directly to the horn. The other terminal on the horn shows it going to the ammeter for power. Also, figure 3-fuse box shows the bottom 2 terminals are for the horn. That is on page 12-4.
I would then assume that for a single horn no relay was used. If a person purchased the accessory horn set up from the dealer then they would add a relay?
If your using a single horn I would just pass on the relay, as long as your horn works ok with 12V put to it. Easy enough test with a battery and a couple of test leads.
Good Luck Don
