get a 2.5" in/out cat. Weld on a new 2.5" collector for your header or get an 2" to 2.5" adapter for your header, cut off the end, weld it on, weld on the new 2.5" flange to match the 2.5" cat. save yourself later for an h22 swap since you'll already have a 2.5" headerback.