The most important thing with horns is to get a beefy supply, something like a 16 gauge wire from the battery through a fuse, to a relay contact and then the horns and straight to chassis., don't try and run the current through a button.
Guzzi in their usual fashion run the wires to hell and back resulting in a wimpy sounding horn, here's another 1978 bike to illustrate my point.
https://www.thisoldtractor.com/guzzi007/schematics/1978_V1000_SP.gifFirst of all it comes from the ignition switch, back through a fuse to the horn and then they take it from the other side of the horns up through the bars and a wimpy wire to the switch and back to chassis.
I guarantee if you wire the horns as I say they will be twice as loud.
The horn relay coil only need about 0.1 Amps.
Wire the horn relay coil as the horns are presently wired, hot on one side other side through the button to chassis.
I kind of equate horns to loudspeakers, think of the heavy wires they use to wire them nowadays.
Another thing with horns they don't like to be anchored down solidly, it damps the noise down, if they come with a sort of springy metal bracket make sure you use it.
The Vox horns in your first picture show this bracket.
