Does this header use the factory mounting locations?

Yes — it bolts directly to the factory exhaust ports and uses OEM mounting points. No drilling, welding, or custom fabrication needed.

Do I need an O2 sensor bung or will I have to weld one in?

That depends on the header design — test-fit before committing. If one isn't present, you may need to have a bung welded in to avoid a check-engine light.

Will I need a new exhaust gasket when I install this?

New gaskets are always recommended for a clean seal. Pick up a header gasket kit rated for your engine while you're here — reusing old gaskets often leads to leaks.