If the flange is straight, the header should seal without a gasket. I have Poston replacement headers, and I used a gasket. Nothing fancy, just a ROL (composition/steel shim gasket) brand gasket.
Make sure you check the flatness of the header flange with a straight edge when you get it back from welding.
I bought a poston driver's side replacement and it was not very flat so I used a fly cutter to bring the flange to a perfect plane. I hear that you can send the headers back to poston if you get a bad one.
welding the header will probably warp it if you don't have it fixtured properly. Then you might need a gasket.
Once I machined my poston header, I didn't use a gasket....just a slight smear of high temp rtv