Longtube headers mean you need a shorty H pipe, will also need to drop the K to install them. I'm not sure about the 3V, but the BBK 2V longtubes suck. Instead of properly clearancing the primaries for the steering shaft they just put a flat spot in one of them.
Best headers for the money for the 3V are EASILY the Dynatechs at $850 for the set and they are ridiculously high quality. They're pretty much the go-to header for the GT500 crowd.
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 ****.