I have a 2000 honda accord 2.3l and I have a header/down pipe with outlet of 2"o.d. and a cat back exhaust with a inlet of 2.5"o.d. trying to figure out if a OEM cat will work with this or what will work

 

welcome to the forum When i installed the headers on my accord, i took my car to advance muffler to make the adapter to my cat so i could swap back to the smog cat for smog check. they also made my exhaust system to match up with the cat and exhaust muffler system.

 

Auto parts stores do have adapters for the exhaust
Try Pepboys. And they also have clamps.

 

Welcome in ,An exhaust shop is easier and simpler over all.