Long story short - I had to reformat my Mac Pro 5,1, which came with an ATI 5870 GPU, and now has a flashed GTX980Ti installed. Not expecting to get anywhere, I tried to boot from the 10.6.4 DVD and low and behold it worked fine. No acceleration of course, but it was able to install, update to 10.6.8, and update using the Mac App Store to upgrade 10.11.4. Not sure if anyone will run into this issue in the future, but I thought I would post my findings in the event it would help someone at a later date. I know I should have had a USB installer of a later OS but under the circumstances, I only had the Snow Leopard DVD. I am sure this is only because the GPU is flashed, but it did indeed work to my surprise! The topic is meant to say 980Ti - Mods please correct.