Those are stock headers and are very restrictive. Get some FRPP stainless ceramic shorties on there. Or go longtubes with a shorty mid pipe. I'd also do the bolt ons and all that junk before I did the TFS kit. You will need to upgrade the fuel system as well. 1, because the stock pump is barely enough for the stock engine and 2 because the 19lb injectors will be too small and are likely shot anyway from being so old.
And welcome to the site, nice fox!
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.