I'm looking at replacement catalytics that are labeled as "exact fit", but the inlet/ outlet sizes differ on all of them. They range in sizes from 2" up to 3". The diameter of my crossover pipe & intermediate pipe is 2.25". Do I want a Cat with 2.5" inlet/outlet so it will slip over my pipes & then weld it into place? Or should I get a Cat with 2.25" inlet/outlet & it will get welded on "nose to nose"? Thanx in advance for your input

 

IMO the best bet is to do 2.25". If you take it to an exhaust shop to get it welded on, they'll have a hydraulic pipe expander. The best fit way would be to cut the factory cat out shorter by a few inches on each end. Then they would expand the inlet of your new cat, and the inlet for the back of the downpipe, slip everything together- and weld it back up to fit.

With 2.5", you'd definitely be having to do a good bit of filling with weld. With the pipe expander, they can get the fit fairly tight to start with so you've got minimal gap to fill with weld.