Replacing the 2 cat setup with one is not legal where I live. I had to have some work done on my Bronco a year or so ago. The guy at the muffler shop told me that by state law (Washington), if a vehicle comes in with 2 cats, it has to leave with 2 cats. In my case, my Bronco only had 1 when I went in, so I was able to just install 1 high-flow cat and be done. If you live in an area where emission testing is done, especially if they do a visual as well, be sure to check out the rules regarding cat replacement. It would be a shame to spend a lot of money on a fancy exhaust system and then fail inspection on a technicality.