340 Exhaust Manifold Help

Hey Guys, I think I can make the 340 manifolds work on my magnum heads. I am cutting 16 gauge steel plates with 340 manifold outside dimensions and a magnum port size hole. Question is, should I use a magnum gasket on the head and a 340 gasket on the manifold? Or should I just not use any gaskets and just use some high temp sealer?