I have the ARHeaders and they fit is great. I also know that Tillman Speed is having a great Christmas sale on Kooks which are just as good. Give chris a call.
If you call Kevin at Clevenger Ford. If you check his hot sheets you will see a better price. That's where I got mine at.
Ford OEM Parts - The Mustang Source Forums
Kooks are some good header, although I have the AR header with catted H and they fit like a glove. If you go with Kooks I think Tillman Speed is still having there special going on. Talk to CR.
I would go with a Tillman Speed tune. I had a 93 custome tune by CR and then we dyno tuned it which made it even better. Also VMP has some great tunes, I haven't used them because Tillmans is down the street form me