You've basically got 20cm exactly to work with. .
Thanks! You know what, I had a brain fade...a PA120.3 with 2 Thermochill shrouds+6 fans wont fit lengthwise unless you move the power supply. Now by my math a 120.2 with 2 thermochill shrouds and 4 fans would fit across with 7mm to spare assuming 2mm thickness of the metal of the shrouds and published spec's are correct, along with your 20cm measurement. But I want a 120.3, so it looks like that double shroud idea wasnt that workable. Thanks for the measurement anyway.