I just recently bought my first 911 and thought I would have no trouble putting my kids (1 and 3) in the back. Boy was I wrong. I was able to fit a Britax Roundabout, front facing, into the back seat, but from what I've researched here there is almost no way to put a rear-facing seat in there. At a minimum you will need the passenger seat all the way forward.
This guy did it:
http://forums.pelicanparts.com/porsche-911-technical-forum/842912-rear-facing-infant-seat-1988-carrera-coupe.html
but he essentially engineered his own restraint system in the process. In my opinion, that's not a safe solution. It has never been crash tested, so just because you think it's wedged in tight doesn't mean it is safe.
If I were you I would explore alternative cars.