Dont worry the rear brake system will look ace the old hose and master cylinder are off, NEW crf450 unit on, Looks trick and a new braided hose to come!

As for the seat unit i didnt have to touch the tank only the part of the fairing that wraps round the top (seat bit) it was hard against the tank which left big gaps at the side and the rear mount holes didnt line up (holes in the fairing where 5mm back so didnt line up) but reshaped the bit that goes round the tank and it fits nice now! still have to finish that off and reroute the wire harness, now that the fairing is on i can see a better route!