Mine were a piece of cake. I had Rhinebeck Ford do it while I was on vacation in Munich drinking really good beer :p. Seriously, it's well worth it. They sound freakin' awesome. I got mine from American Racing Headers in L.I.
I remember seeing those when I was a kid. It's an interesting variant of the Mustang and obviously very rare. Don't know of any in my area but if I see one I will certainly send them your way. Post some pics!