What I encountered was this:
I run a 5k in a KE18.
Original water pump was likely from the original 3k, as it was a very short neck with a shallow pulley. This allowed the fan to fit between the motor and original rad. When I replaced the pump with a unit from a newer 5k motor, it was a much longer neck on the pump with a deeper pulley. Also, the four mount screws were a slightly larger radius. This meant i had to swap fans too. BUT, I then ran shit out of room, and as TasRX stated, it ate its core when the engine shifted forward on a 2nd gear downshift. This caused the fan blade to nick the bottom of the top tank, folding a shard of plastic inwards and mincing my core.
What i have done now is fitted a KE70 rad, removed the mech fan, and fitted a front mounted thermo fan. As i was still shit out of room for even a thermo fan to squeeze in there.