Exhaust fitment, particularly the Y-Pipe.
 

I recently changed converters. The new outlet is 2.5" but does not match the stock Y-Pipe inlet of 2.25". I really want that to match up, while keeping the mid-pipe back as stock, but am encountering some issues.

Any Y-Pipe I'm finding online with a 2.5" inlet has a 3" outlet. Would that work with the stock exhaust? I'm thinking the 3" outlet would be a bit much for the stock mid-pipe. Please correct me on that if incorrect.

I have also considered a custom Y-Pipe with 2.5" in and 2.5" out. Would that work or is there harm in not widening the outlet from the inlet?

Any advice on this topic would be extremely helpful. I have found numerous threads with similar topics but have not gotten the specific answers needed. Thanks!

You can try a reducer to match up the converter outlet to the stock y-pipe inlet, and that should solve your issues. Here's an example:
Any competent exhaust shop should be able to weld it up.