Rip out all the crap from the Power Mac G5! Really, all of it! In the bottom of the G5 is the PSU in a detachable box. Take the box out and remove the contents except the power connector. Drill a couple of holes in it and bolt the two ring core trafos to the bottom. Connect the input to the trafos in parallel to the power connector. There should be some holes in the PSU case that the output of the trafos can go through, so stick them out there and re-mount the PSU box in the G5 case. See pictures.

Assemble the amplifier circuit boards. This is beyond the scope of this guide for now. (Really 'cos I assembled them ages ago and never took any pictues :( ) But have a look at the ashematics and layout for the PCBs: https://bitbucket.org/intelligentagent/amplificent

On the top of the PSU box, mount the amplifiers. Make sure you use spacers to get the PCBs off the metal sheet.

Mount the Arduino TRE on the back wall of the G5 cabinet. This where the motherboard used to be, so there should be plenty of nuts attached to this wall. Obviously the attached nuts will not fit to the Arduino, so use an acrylic plate to act as a buffer between the mounting holes on the TRE and the mounting holes on the G5 case.