Looks real Good Crio.
If I might make a couple suggestions...
a) Make the PSU area larger as not all PSU are going to fit in the that configuration and you also do not want to impede air flow through them.
b) A PCI Expansion bar would be a good idea especially for the GPU LN2ers
A couple of long threaded rods mounted either just by the PCI area, or on either end of hte motherboard with a piece of strap steel hanging between them. If you run it the whol length of hte mobo... you will need to use thicker metal... but you can always use a third rod in the middle for strength.
c) Doing the expansion support bar the whole length of the board also would allow you to make fan mounts around the CPU socket area for FET cooling instead of just hanging them off the POT.
d) wing nuts to hold the top to the bottom for faster access to the HDDs. (Or use a "tool less" HDD mounting design
)