Cramming power into tight spaces is a specialty of mine (see my PSITX and Millennium Falcon builds), so I've got a few pieces of advice:
-Use 2.5" drives. You can put 4x 2.5" drives in about the same space as a single 3.5" drive.
-Ditch the water cooling. It's bulky, and in low-volume environments you don't need a lot of CFM to get good airflow if your ventilation is well placed.
-You don't need a GTX 780 for a SteamMachine. Sure it's fun for bragging rights, but if the idea is to be small/portable or hook up to a TV, you won't be gaming beyond 1080p. Get the GTX760-ITX by MSI
. It's the same length as the motherboard and will give you a lot of space with no tangible performance loss at 1080p.
-Put the PSU inside the case. The above recommendations should give you enough space to do so.
I'll also note that the ASRock motherboard you have (which I am a fan of) has an mSATA slot. Use it. You can get a Crucial m500 mSATA SSD for cheaper than most SSDs (240gb for under $130, 480gb for $230). It will save space and increase performance much more than a GTX 780.
I hope this helps!