I have a 1997. 18" were not approved for 1997. 1998 was the first year. I do not even think Porsche had 18" wheels with the proper offset in 1997. What I remember is that the BMW Z3 came out with optional 18" wheels and then all of a sudden so did Porsche.
There is a guy on rennlist with a 1997. He has 18" wheels and a 3.4 conversion. Don't think Porsche approved a 3.4 conversion.
http://www.renntech.org/forums/index.php?showtopic=3130&st=0&p=16487&