Nice work! I've done the same with a G5 and i have a G4 in my basement too and also a bunch of hardware, i'm also planning to do that with it
You are doing a good work
I have a PowerMac G5 Case too, but I have no time to keep my hands on it :\
Finally, with a cardboard box I found a provisional solution. When I close the case, the CPU heatsink is in front of the fan.
I was measuring the area for my 2x120mm radiator. In the bottom of the case, I've a 290x130mm area, completely clear from mounting points. I think I should use an Alphacool NexXxoS ST30 Full copper. It has a 15mm spacing between fans, so it's a little bit longer, perfect for my non-standar 290x130mm area. The radiator measures 279x 124x30mm.
For the CPU block, I'm planning to use a EK Supreme LTX Acetal Black. But, Can I use 90º fittings for the block?
The area for the radiator: