Dear Ralf-Arthur Rebholz,
Sorry that we are afraid the distro plate of PH518 is not compatible with the 280mm radiator on top and a 420mm radiator in the front. But you can use it with a 240mm radiator on top and a 360mm radiator in the front. Such as the kit BPTA-WDPPH518-KIT shows.
Any questions, feel free to contact us.