Just thinking out loud here. If you are ok (?) with changing the orientation of the horn you want to use (pointing down instead of forward), you could bolt the existing Z-bracket to the firewall and the horn to that bracket. It might place the horn forward enough to clear all the things on the firewall and in the engine compartment. The horn doesn't care which way it's aimed to function. That heavy brace and orientation change should solve the "floppy" problem mentioned by Kevin.
