At the end of production for the first Generation new MINI came the GP. It was the fastest MINI to date at the time and had some interesting aero tweaks including full length under-body panels to smooth the airflow under the car. Those panels run at least $200 each if you can find them, but there is an alternative that is almost as good (but also at end of life.) The second generation (R56) Cooper S had optional underbody panels which were 90% the same shape as the original ones and only about $60 each. Fitting them is fairly straight forward, but does require some modification of the panels and perhaps fabrication of one bracket.
The first Gen cars have two of the necessary mounting points already attached. Just remove the air flaps ahead of the rear tires and the panels slip on. Once you work to the front of the car you will see the bits that need to be trimmed away to make them fit. You will need to drill six new holes in the floor pan to attach the rest of the mounting points, but don’t worry, there is nothing in the way of the bolts on the other side. On the passenger side, you will need to use a thread cutting tool to capture a bolt for the outboard side and the bracket for the inboard side. On the driver’s side you will need to thread the outboard side then figure out how to attach the bracket. I had it welded in place. Now move to the front. You do not want air washing over the leading edge so get some 1/8th inch thick ABS plastic, bend it to fit and attach using screws or pop rivets. That’s it. If you have a lift, it could be done under 2 hours. If you’re working on jack stands like I was, plan to double that and add a trip to chiropractor to the list.
The first parts list shows the R53GP part numbers. The second one is from the R56. The next three are before, after, and the leading edge modification. The last photo is the Rennline Skidplate we installed the week before.