On the RIV-Gene that you pictured (I have one), you can use the Epsilon on 4 6990's: you just have to also get the single slot IO shield as well to replace the stock one. You'll be better off using specialist fittings for block connections (Look for SLI/Crossfire fittings/bridges) to get them all plumbed - and I'd plumb them in parallel - and you should be good to go. Just be sure to put a seriously beefy PSU in that case.
My only concern is, and I have no way of checking this without emailing Swiftech and asking, is if they have enough room to fit a fitting between the blocks: Usually you have a empty slot between cards: with going slot to slot, you'll only have 20mm give or take - can't recall - of room between each PCB. The block takes up 15mm off of the surface of the PCB, so you are fine there. It all comes down to the fittings.
EK also makes a 6990 block, that while it also has a possible fitting interference issue, is a little thinner at 13mm off of the PCB. Again, be worth emailing EK to see what they would recommend since they also sell a single slot IO shield.
Why I was wishing you was talking about the 7990: because I do know that you can run EK blocks on single spacing: and they make a special bridge block to link them together.
(Check the professional VGA Block section: the S10000 is naught more than a 7990.)