Will this fit my 2.5" downpipes and cat-back?

The 2.5"-2.65" OD range covers most universal tubing, but measure your collector outlet and cat-back inlet first. If either is undersized, a stepped coupler bridges the gap.

Do I need to weld or bolt this in?

H-pipes are typically welded for a permanent seal and structural rigidity. If your setup uses V-band or clamp-style couplers at both ends, bolted assembly works, but confirm your collector and cat-back flanges match first.

Should I paint or coat the bare steel?

Bare mild steel will surface-rust if exposed to moisture. High-temp paint, ceramic coating, or stainless steel wrapping all extend life. Paint and cure per manufacturer instructions before bolting into heat zones.