The legality would depend primarily on which state you live in. In Ca. we can't do anything to alter the installed emissions on a vehicle. When I had my 2.6 I lived in a county/area that didn't require smog inspections after the initial issuance. Having my own shop and personel let me play however I liked. Now, everything has to conform and there are no CARB (Air Resources Board) approved items I can use. Some states don't care what you do. To put the Weber on some of the emissions related systems have to be changed and/or, abandoned. If you have annual or semi-annual smog inspections where they verify equipment you either have to conform or put it back to stock for every recertification process.
Is it a hard install - no - not if you have been doing your own work from time to time and are saavy. There's a huge number of people that can help on this board.