Back in the past I added a Magnaflow highflow catalytic. Having a Series 2, there was some concern about the rear O2 sensor having a mounting location and I ordered the cat with a rear bung. As you can see from the picture the Magnaflow'* are much smaller than our originals and there'* no need to cut anywhere near the stock O2 location. We can also see the hole in the catalytic.
Seeing that there is something missing in front of the cat we see the studs that are part of the flange usually welded to the cat. Four small nuts and off comes the down pipe where it mounts to the catalytic.
Original down pipe
Stock and 3" downpipe side by side.
Inside the down pipes, stock is 2" and 3" is just that..3"
Thanks again Tom..