ok wait... i may be able to answer my own question.
on the left is my greddy header and on the right is my old h23 header (or downpipe or B-pipe or whatever you want to call it...)
so... my common sense (meaning, im going to need a second opinion at least lol) says that since the end of the header is the same, this means i should buy an h23 catalytic converter correct?
then i run into the question of... what brand should i buy (dont say testpipe b/c that is not an option for me)