I'd still like to see the specs in the op

Specifically card mounts built in to run the cards with no PCI brackets and adjustable mounts to evenly space 3-6 GPUs.

I like it! Dumb question, tho: which end are you referring to when you say "front" and "rear"? I kind of assumed that the PSUs and MB connectors would be at the front, and the PSUs would blow the hot air out the back, but your wording makes me not sure.

So for the 4U version, you would have everything the same as pictured, except:

-GPUs would be centered vertically, to allow for riser depth on the bottom, and PSU power plugs on the top.
-GPU backplate would be swiss cheese mesh, to allow for airflow.
-3 center fans would be upgraded to 140mm sizes. Will these be too tall for proper cable management, with risers below, and PCIe power above?
-Most of the metal case on the front (between PSUs and MB) and sides (before the fans) would be vented swiss cheese.

That actually sounds perfect. How much are you looking to charge?

It also looks like you could put a large fan between the PSU's on the front though it would have to be a hell of a high pressure high cfm fan to supply the three 120-140's in the center so open air may be better.

Those PSU's should exhale onto the GPUs or have a vent to push the air above or below the gpu's

The issue I see is that you are drawing heat from the GPU's and pulling it into the PSU, It's likely better than my solution which spits PSU heat into the GPU's but it's still going to heat up the PSU a lot.  Especially if you are like me and load up PSU's to 90-110% 24x7

Very close.  But that beast looks to be 6-8 Rack units which nets 50-75% rigs per rack compared to my solution.  Also you have PCI brackets still on the cards for mounting and power below the cards which is a terrible idea for heat.

*And why would you put all that hardware in a rig and use a 3.5" non SSD hard drive?