I  initially tried to mount the Xbox motherboard to the metal case with short wood blocks.  Some of the pics here you can see some of the blocks I used.  After getting about halfway into this project space started becoming a big problem.  I ended up looking to JBWeld for the answer again.  

I taped over the existing mobo mount points so nothing could touch the Xbox guts and get shorted.  I took some old brass(?) motherboard screw in mounts, and cut off the screw in bit.  I then JBWelded those mounts to the Antec case, thus giving me the exact mount points I needed for the Xbox motherboard.  

It's not the most high tech solution, and given more supplies and equipment I am sure I could have done something slicker, but it worked perfect and still works to this day.  Not one mount has popped off. 

After deciding on tons of mods I wanted to do to this, I eventually decided it had to have some fluffy shiny things on it too.  A LCD display was totally and utterly useless, but looks cool.  

After putting all these mods and upgrades into the system, as well as adding internal wireless controller modules and various other stuff the box got pretty tight.  Cable routing became very important and fan size and placement was imperative as well.