Does this header come with gaskets and mounting hardware?

The assembly itself is included. You'll need to verify gasket and fastener condition before installation — have new gaskets on hand, and refer to your service manual for the correct bolt specifications at each mounting point.

Can I weld or modify this header after purchase?

Stainless steel can be welded, but have any modifications done by someone experienced with the material. Heat cycling can affect the stainless properties, so work should be done carefully to avoid cracking.

Will this header require any exhaust system changes downstream?

Verify the outlet diameter matches your existing downpipe or cat section. If diameters don't align, you may need adapter piping or a compatible exhaust component to complete the installation.