Jamie,
I ran 215/60VR-215s and 235/60VR-15s on stock-fendered '73 for years on 6 x 15 Fuchs ... Pirelli CN-36s!!! You may have to settle for a variation in available sizes with the limited selection of high-performance 15-inch tires, such as 215/65ZR-15, or 225/60VR-15, or 235/55VR-15 ... but there are still some available.
http://www.tirerack.com/tires/Sizes.jsp?make=Pirelli&model=P600%2CP600+M%2BS
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Warren Hall
1973 911S Targa