I just di da search for a project on how to install a y-pipe and i didnt find any.
you don't remove all the cats if you install the y-pipe, just 2 pre cats.
If you do the y-pipe and the high flow cat you do remove all the stock cats
and you replace them with the high flow cat, it has the 02 sensor bung that you
screw your 02 sensor in so you don't fail inspection. You can do this with a lift.
George C has a TT and shackle lift and he has the y-pipe and maybe soon to come
the high flow cat.
Paul H