Does this tubing need to be welded or can I use clamps?

This is raw tubing — you'll need to weld it into your system for a permanent, leak-free connection. Clamps alone won't hold exhaust pressure safely.

Should I paint or coat this before installing?

Raw steel will rust quickly once exposed to exhaust heat and moisture. Most builders either ceramic-coat it beforehand or use high-temp paint before final welding and assembly.

Will this fit my stock exhaust manifold outlet?

That depends on your manifold's flange diameter and port sizing — this is raw tubing without adapters or flanges. You'll need to verify your manifold outlet matches 2.25" and source any transition pieces separately.