Power supply for usb devices
In short, yes. If you go to the Specifications tab you can get the output specs:

+3.3V@22A, +5V@30A, +12V1@18A, +12V2@17A, -12V@0.3A, +5VSB@3A

A single Raspberry Pi will theoretically draw 700mA at most, but generally peaks at around 500mA. (source) So, if you have 700mA (we may as well design for worst-case) drawn by 40 Raspberry Pis, you'll have a net current draw of 28 amps. This means you could use the 5V@30A rail from just one of the power supplies to power the entire cluster! However, you'll still need a way to split that power 40 ways, and if you want to add the fuses go ahead; if you can't find 750mA ones, 500mA slow-blow types should work (assuming you want one fuse per node). If you simply want a mains-side fuse (I wasn't sure based on your post-may as well cover all the bases), I think 1A (at 115V) would be sufficient.