Technically there is no 1KW transom mount transducers compatible with Accu-fish due to all the turbulence normally experienced. Only the thru-hull units listed in the manual are considered compatible.
When selecting your transducer in GP1870F, if the 1KW transducer you have is listed in the menu, select it. If it is not one of the listed transducers, pick the "1Kw" setting. The FCV628 is a 600w unit. There is no 1Kw settings in this sounder. It is possible to run a 1KW transducer at 600w with good results for the extra element sensitivity.