I have California smog 280z was wondering how to use a federal smog (no smog) header on it. emissions is not a problem for my state.

Are you going to replace the exhaust too? If not I'd suggest talking to an exhaust shop about wielding one to your current setup (unless you can do this yourself of course).

MSA has a pretty sweet kit that's all bolt on if you choose to get new exhaust too. It's the 6 into 1 with the premium exhaust kit. Current I just have the premium exhaust kit they offer and love it, hopping to get a 6 into 1 in the near future.

I don't think there is such a thing as a federal smog header. Just be sure to get the proper header for the cylinder head that you have, some are square port others are round. Stock 280Z should have n42 head which is square. From expierence some smog inspectors will fail a car with a header regardless simply because it is not stock and does not matter if emissions equipment is hooked up or not.

Plan on replacing with 3 to 2 and twice pipes and switch exhast. But there is a pipe on the firewall side of of the intake manifold that goes down and does a 90 to the exhast manifold. SC has no inspections

Sounds like the E.G.R. valve, both types of headers are available. If you are not worried about emissions get the one without the E G R hook up and bypass the valve.

All three of my 77's have N47 heads, which have round ports. Be certain you double check the head and port before ordering a header or you may not get the one you need. As I recall, the Cal emissions motor have an altitude compensator and a catalytic converter but nothing else. No smog pump or O2 sensors in the exhaust (until you get to the ZX years). My header came with an O2 boss and a plug if it didn't use the sensor.