I have this same machine, and have been struggling to get High Sierra to install, and wonder if its a related issue.
My Mac Pro is running Yosemite, but has a new SSD in the second bay that I am trying to do a clean install of High Sierra on.
When I first ran the installer it did ask to update the firmware, and when I look at it, it says its MP51.0084.B00 which seems to be the current version.
But—this may be a stupid question—is the firmware related to the drive, and does the SSD need to be updated seperately? If so, how do I do that?
I have tried every other possible solution, from reformatting the drive with an external HS utility to installing from an external USB, and the install stalls almost immediately no matter what I do.