I would prefer a setup that has one ai flow direciton, possibly forcable from the front. The reason is that when yo put some of those together, obviously, you get into a problematic area with the heat.
The classical cold / hot isle approach helps a lot - sucing hot auir and sending it out (not cooling it), while injecting cold air at the front of the computers.
Seems 6990's push the air in all directions (not good for that)
Any case suggestion, preferably simple box, so it can be stacked nicely?