Why would you want to fit the CU II cooler from a 280x to a 390x? The coolers on 390x are larger and more capable, usually with 3 fans and not interchangeable with coolers from other smaller cards.
Cards can be either a refference design, or in the case of Asus Direct CU models, Gigabyte Gaming or G1, EVGA Super Clocked, MSI Twin Frozr, Sapphire Nitro or Tri-X - they are non reference cards. This means the card layout is different and developed by each manufacturer.
Simply put - it will not fit.