Seriously, 6 R9 290X in one board is overkill and expensive to build by the time you add in all the power supplies.
These 290's are extremely power hungry and heat up like crazy. Even if you manage to get it up and running stable (unlikely), it will be super heated and probably break down or have issues within months.

Just do some cost analysis for yourself, using a standard cheap Asus or AsRock mobo which sells under $69 and can ran 4 GPUs is a much better option in my experience. Saves you money, troubles, headache, time fiddling around and most importantly, has been proven by many to run stable over extended periods.

My 3 GPUs setups have been running smooth for over a year now. And I'm very certain that it's cheaper per Mhash than building 6 GPU rigs for the same cards, no matter what card you use, be it R9 280x, or 290 series.