Hardware review
Is there any problem with using an older IBM x3850 M2 server as a hash cracking rig.  I have a friend taking one out of production, and has offered it to me.  It has 7 PCIe X8 slots and duel 1440W power supplies, but they are redundant so I believe I am limited to one at a time.

Will a PCIe x16 card physically fit in a x8 slot?  I know I can use PCIe x1 risers if I need to.  Do you think I can get away with using three gtx 1070 cards with power supply or should I stick with two?
An x16 card won't fit in an x8 slot. 1070s don't draw a lot of power, but this entirely depends on the draw of the rest of the hardware in the chassis. Get a Kill-A-Watt and baseline it.