Time to create a supercomputer!
By what method will you be integrating these 70 RPi SBC into a Beowulf cluster? I have thought about building a cluster with a few Bitscope Blade Quattro but at little over $50 a unit, the cost to interface more than four RPi could be quite expensive. It is a great idea. People have used the RPi in clusters before but I don't believe anyone has attempted to use as many as 70 RPi. There are a few Linux applications available to run a cluster. Opensshserver and MPICH are a few that come to mind.