I installed a muffler from a R56 MCS on my R53 the other day...

...I also used the R56 muffler hangers...

...there are pre-drilled holes in the rear "frame rails" and the hangers just slide in from the rear as they do on a R56...

...view from the rear looking in through the grille area...

...of course with the straight R56 exhaust, the battery box has to go bye-bye...

...the battery is an Optima Red Top mounted on it's side...

...somewhat finished battery install...

...sitting back on the ground...

...side bumper view...

...I did have to shorten the tail pipes so the tips didn't stick out to far.
I had the resonator removed from my JCW exhaust a while ago, it will going back in because this setup now is very loud... and it snaps, crackles, and pops like crazy.
The car feels much more snappy and quick now, the R56 muffler is apparently fairly free flowing.