Mounting Mach 460 tweeter

I assume you're talking about the entire Mach 460 tweeter pod, not just the tweeters. If that is the case, you will need a new door panel. Mustangs w/ Mach 460 had diff. door panels to accept the tweeter pod...or you could just cut into your pre-existing door panel if you wanted.
 
the pod mounts to the door via factory locations. If I recall, there is only one or two screw locations that actually mount the plastic tweeter pod to the door shell. Just take your tweeter pod and look for the pre-drilled holes and try to match it up w/ pre-drilled holes in the door. That's how mine was. It's really a simple process, but if your car didn't come with the bolt locations, then you may have to drill some yourself.