I think you should spend on a higher quality PSU. Although technically your build may work with 300W, you will strain it to the max. You may also go over the max power of a given rail, since 300W total does NOT necessarily mean 300W for the 12V MB and GPU rail. If the 6-pin (75W) PCIe connector is an indication, transforming this to 8-pin (150W) may or may not be a wise decision. Just because it fits, physically, doesn't mean it can provide this electrical output.
For example, in Techreport's review of the 1070Ti the power consumption in a specific game would reach 250W without any overclocking (http://techreport.com/review/32766/nvid ... eviewed/13
). Other sites, using slightly different configurations and games have shown system load at 300+ W. Again, without overclocking, which increases consumption much more than linearly (actually, I believe it's slightly more than quadratically, if you increase voltage).
My recommendation would be to go for a 500W+ PSU, preferably one from a top-tier vendor (Corsair, Seasonic etc). The Seasonic Focus Plus Gold ($79) or Platinum ($99) are good choices, for example. Most PSUs have optimal efficiency at the 20-80% range, so running at 50% would be great for the longevity of your build and for the quality of the electrical output (noise, ripple, heat etc).