I have not found a rackmount chassis with more then 4 external 5.25' bays, also i believe it will be difficult with only 600 depth. I am using this monster:
COOLERMASTER STACKER STC-T01 see
http://www.systemcooling.com/cm_stacker-01.html
I mounted the rear-fan outside, 2 power-supplies inside, 2 motherboards with spacers on top of each other, now it holds the USB-NAS and the smootwall firewall, and has space for future drive expansions. I plan in future to replace the motherboards with mini-ITX, then it should hold multiple motherboards for my complete network. The only issue on the case are the alumium sides...they are a bit flimsy.