Thats what i did--i drilled a hole in the power supply and tapped the power leads to come right into a main block--did the same thing with the monitor--this eliminates like 8 or 9 feet of cable as well as the space for the plug ins--also, if you're using an AT power supply and dos--its 1 button off, 1 button on--for everything.
To get everything to fit, i would suggest laying it all together, keeping in mind you can work in 3 dimensions if need be (screw hard drive to side wall) and see what you come up with. The dell optiplex board i used was ideal as it was thinner than it was long so it layed on the bottom perfectly.