so...what year camry do you have? and are you saying that the diameter of the piping is too small to fit properly onto your stock piping?

and what's wrong with a universal cat? simple welding job

 

OK I have changed the Y pipe to bosal a week ago . when I got the bosal converter the outter diameter of bosal Y pipe is close to 2.5 inch but the inner diameter of bosal direct fit converter is only 2.25 ????????????. I was thinking of using the pipe expander but then it might crack the weldings at the mouth of the converter what a bump.
My technician friend advice me that the direct fit is better than welding a high flow universal in terms of rigidity ( support) n of course not in terms of air flow.