No one makes headers that will fit my 70 GT with PS, column shift, with a FMX transmission. My choice is to replace the column shift with a floor shift, which I am planning on. It just seems weird.
I don't see where you told us what motor you have? If it's a 302 then Mustang shorties will fit it, and the rest of it can be done by any muffler shop. Again you'll need a floor shifter of some kind, because there's a countershaft directly in the path of the exhaust outlet.