Can you post some close up pics of the area around the carb? Gotta either find something to hook to, maybe drill a hole somewhere in the tin or plastic work ect. Or you've gotta make a bracket to hold it and bolt it to something. I haven't worked on too many V-twins but every engine is different.
I would get the spring you want to use and hook it to the throttle, then see where it looks like the best place to put it is. I've even made extensions from a metal coat hanger to hook stuff in some cases. You can bent them around with cutting pliers or whatever.