Thank you again. I got this working yesterday by downloading High Sierra and updated the firmware using my old Radeon HD 5870 that came with the mac pro.I was already on the latest firmware that was installed during the update to the latest High Sierra. mp51.0087.B00
I believe you need at least the mp51.0085.B00 to install Mojave.
Firmware update was fast and painless by the way...
P.S. You have to have a mac EFI GPU to do the firmware install. I used one of my flashed GTX 680's to do the firmware install.
everything worked so well with RX 580 and the Mojave seems very stable as a beta.