Thinking of going with this one because you can reuse the factory cap. What are you guys using?
http://cgi.ebay.com/ebaymotors/_Car-Truck-Parts-Accessories__MOROSO-63768-EXPANSION-TANK-2005-2008-FORD-MUSTANG-4-6_W0QQitemZ290262710048QQadiZ2865QQadnZCarQ20Q26Q20TruckQ20PartsQ20Q26Q20AccessoriesQQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?_trksid=p4506.m20.l1116
http://cgi.ebay.com/ebaymotors/_Car-Truck-Parts-Accessories__MOROSO-63768-EXPANSION-TANK-2005-2008-FORD-MUSTANG-4-6_W0QQitemZ290262710048QQadiZ2865QQadnZCarQ20Q26Q20TruckQ20PartsQ20Q26Q20AccessoriesQQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?_trksid=p4506.m20.l1116