Exhaust Design

[Billy Mild]

I recently acquired a set of used SW headers. At some point the collector was changed. The standard 3" collector was removed and a 4" collector was welded on. After doing some research on reducers it appears the pickings are slim for a 4" to 3" reducer that also has a smooth transition. I would like to put the headers on myself then drive the car to an exhaust shop to have them connect it to the rest of my exhaust. The reducer is what throws a wrench in this. I would hate to use a step down reducer that would disrupt exhaust flow.

My exhaust is stock, minus center resonator being deleted, and axle back Borla installed. It sounds good and doesn't drone bad.

What would be the best thing for my overall exhaust design, change my collector on the header to the SW 3" collector, or find a good reducer?

I would like to run cats on this car. I am looking for C6Z cats right now.