Herman,
not sure of your location, but Yarra Valley Exhausts in Lilydale are VERY good.
They do a lot of mustangs...esp given the number of mustang repairers in the area.
They recently did a boss 302 for me. $370 to me for a dual 2.5inch system.
If you want something that looks technicaly correct, then tell them to ask me for the correct diagram for your car. they will make it look right.
I have been using these guys since long before i opened my own shop and have always been happy with the result and prices.
Cheers
Jas