Note that even with the fuel tank underframe secured to the chassis, there is an access hole for the horn mounting screw and lockwasher. In other words, you don't have to remove the fuel tank to install or remove the horn.
Remember that it is very important that the relay bracket is insulated from the chassis. As mentioned in the article, the diecast fuel tank part usually comes with the battery plate already secured to it.
Parts diagram is on the left. Above is the wiring diagram.