All you really need is this T5 harness to connect the reverse lights to your T5. I suppose you could cut into the red bypass loop and activate the NS too, but 64-73 Mustangs originally had no NS on manual transmissions.
I have a T-5 and my neutral safety switch works, it was simple to fix too, but it was about 3 years ago so I cant remember exactly how I did it. I do remember using a 14.4 volt battery out of a cordless drill as a powersource to trouble shoot it while I was figuring it out though.