Welcome to the forums..You would need an adapter cable to do that..Or..An 8-pin PCIe connector that you can take the terminals from the 8-pin CPU connector and put them in the proper position in the 8-pin PCIe connector..Or..If you will have other unused connectors such as dual 4-pin Molex or SATA or 6-pin PCIe, you can easily find the adapters for those too if you search around..Graphics cards come with an adapter too.
 
See GPU and CPU power connections at Overclock.net for more information..The GPU 'Sense' wires are just ground wires.

Make sure 1600W is enough to power the 1080 Ti's and all the hardware in your system..PSU calculators here and here..You'll get a PSU 10% discount code (with conditions) in your email for using the EVGA Power Meter.

You're welcome..Good luck with your new build. 

I seriously doubt the claims about the "mining" GTX 1080 variant (P104-100) managing 60 Mh/s on ETH mining - but it should draw substantially less power as it will have no video circuitry and will probably disable the video-specific parts of the GPU (similar to the BIOS series TheStilt made for the R9 290 back in Litecoin mining days, that disabled most of the video-out parts of the card for significant power savings and cooler operation) and bios mods MIGHT make it faster than the consumer cards - but more than TWICE as fast is balogna, the latency on GDDR 5X is a substantial hit on ETH performance for the 1080 (the 1070 is significantly FASTER, despite having 1/4 fewer cores to work with and 20% lower memory bandwidth, because GDDR 5 has a LOT less latency).
 The cards SHOULD be more efficient than "consumer" versions - but there is some question as to if the actual price to an end user will meet the rumoured $350, and substantial question as to how easily a "small farmer" will be able to get ahold of ANY of these "mining specific" cards.
 
 The GTX 1060 variant (P106-100) has a lot more believeable claims - and there are sites already making it available for pre-orders (at a noticeably higher price than the rumoured $200 MSRP).
 
 I'm not inclined to call these cards vaporware, given the several pre-order listings, but I suspect a fair bit of the rumours about them aren't valid - especially some of the claimed performance numbers that make ZERO sense.
 They SHOULD be substantially more efficent, and SHOULD be able to demonstrate small performance gains if they have BIOS modes to optimise them AND if they have properly-chosen memory - but more than double the performance is .

Oh, I have no doubt that the sellers will try to get as much as they can gouge, err, profit out of these "mining" cards while they can.
But to be picky, it was the Litecoin mining craze that put AMD in the same position in 2013-2014 - Bitcoin had long since moved to ASIC by that point, and the earlier Bitcoin GPU days didn't engender enough card sales to create a shortage (Bitcoin was still SMALL at that point).
 
The difference is that it's MULTIPLE coins benefiting from the recent price jumps, and NVidia is competative to even SUPERIOR at mining some of those coins, and it's in the performance AND AT CURRENT PRICING it's even price competative per hash on the GTX 1070 and GTX 1060 for Etherium (the BIG GPU mover right now).
 
 Unfortunately, the big ETH mining farms seem to have realised that - GTX 1070 availability has gone to "very scarce" over the last week, almost as hard to find them at all as the RX 470/480/570/580 and the price is climbing into GTX 1080 territory when you CAN find them.
 
 The current craze has also impacted older AMD cards that are good at ETH mining - try finding a HD 7950/7970/7990, R9 290/290x/295, R9 390/390x, or any Fury varient for much under $500 any more.
 If they weren't so highly profitable, I'd be tempted to sell *my* R9 290s - I could easily get twice what I paid for them, perhaps 3x - and they've already paid for themselves anyway through ETH mining in the year-and-change I've had them.