I'm in the process of finishing off adding an A1200 motherboard to an A1000 case. I'm using an external power supply, and there is plenty of room. So, if a similar thing is done, an A600 board will fit fine. Even if you still use the original power supply, I am sure it will fit fine as well - I just did a quick visual check comparing A1200 keyboard width (minus the keypad - which is roughly the width of an A600 motherboard) to the available space in the A1000 case. I realise that's not a specific answer to your question about measurements, but I hope it helps.
Also, over at AmiBay I'm currently selling a PC to A600 keyboard adaptor that may help you with matching up the A1000 keyboard.
Good luck!
