you need to change the side black parts of the bumper aswell and not just the bottom bit! the cooper s ones are lower than the one or cooper so they dont line up at all!
they just clip on though with a screw in each corner holding the bottom trim to the side trims!
look at my gallery i have a scorpion twin on my cooper, me and my dad fitted it in about an hour and half with a bit of modification!
theres no need to be drilling through the body or anything, so dont let your exhaust fitter think they need to! this will affect future value and the fact that it can be put back to original easily! you can modify a bracket to hang on the bottom of the chassis with bolts through holes that are already there!