try if you can use the onboard GPU for the regular video output, then all your 1070s will perform full without loss on 1st card 
if bios complains, look if you have somewhere "Windows 10" enabled, disable this and just enable Bios itself to UEFI only - thats enough for running (just during installation i'd to enable always Win10 and remove all GPU's, then disable Win10 and add all GPU's and leave the onboard GPU as primary video output)
if bios complains, look if you have somewhere "Windows 10" enabled, disable this and just enable Bios itself to UEFI only - thats enough for running (just during installation i'd to enable always Win10 and remove all GPU's, then disable Win10 and add all GPU's and leave the onboard GPU as primary video output)
All my systems are 7x or 8x GPUs - I'm not sure if the asus Z270-A will allow you to keep the onboard GPU running with that without causing another issue?
with additional 7x GPU's it should work, not sure about additional 8x GPU's - i have 6x GPU rigs not to take any risk on the PSU