The 987 18" rears are 9" wide with an offset of 43; stock 17" 986 rear wheels are 8.5" wide with an offset of 50. There should be no clearance issues on the rear, the wheels will stick out an extra 1/2 inch. Likewise there should be no clearance issues on the front with the smaller offset number.
Visit this site to confirm:
http://marksink.com/tire_wheel_offset/offset.html