build a lug and either weld or braze it on. braze has a lot of advantages if the part is designed right. look at the braze-ons the bike is built with, you can duplicate that idea with 2 L brackets tacked together to make a T. form the 2 flanges around the tube.
as for disconnecting the battery
when welding, I am of the opposite camp. the battery
acts like a big capacitor... thats one of the things it does in the system. it soaks up spikes and transients. if you are worried, disconnect the fancy systems... CDI, ABS and such, but be aware that these systems often have grounds that do not disconnect with the main connector... that leaves a potential path, especially with the battery
I have arc welded on cars and motorcycles many times (stick and MIG). the key is to get the ground lead close to the work area
the guys in Long Way 'Round melted wires, nothing to do with the arc other than the heat