that will work fine, you may have to connect a "dummy load" to the +5v and/or the +12v - could be anything like a light bulb or resistor. I used resistors on a breadboard to make roughly .5A on each. They get a bit warm so remember that when you grab 'em (you only do it once, lol).
Another way, if your rack has a shelf on it get some 5.25" external ODD cases like these
SABRENT EC-525K http://www.newegg.com/Product/Product.a ... 6817366013
BYTECC ME-535U3 http://www.newegg.com/Product/Product.a ... 6817145090
and they will just use a power brick instead.