The changed clock rate of the P475 causes a change of its Gestalt code (kind of an identifier of a given hardware setup). The installer application checks the Gestalt code of the computer prior to attempting a system installation. As the changed Gestalt code is not on a list of supported systems, the installer refuses to cooperate -

Fix: either throw in a hardware switch and change the speed setting circuit in a way to activate 25 MHz or 33 MHz alternatively or else use some software to spoof a Gestalt code of your choice. The control panels Who am I? or Whish I were are available for this purpose.