I see what you're trying to do, but those tape adapters are not really the best solution. Check out the DICE FM Modulator I installed in my Boxster in the article here:
Pelican Technical Article: Changing Your Boxster Stereo Head Unit / Installing Boxster Ipod Integration - 986 / 987 I think it's a much cleaner installation.
That said, to answer your original question, yes the radio from a 2000 996 with a tape player in it should work fine. My wife's Boxster has the CD Changer in the front trunk and the tape player behind the faceplate.
-Wayne