Well, I shorted the CMOS clear and got back to the POST screen. reconfigured the BIOS as MacMan had described.
Replugged my monitor to the internal GPU. I get the Clover menu and if I choose my OSx startup, it goes through the first half of the Apple process and then stops. Screen black, no disc activity and then a reboot.
I pulled the RX580. Reboot OK through Clover and first half of the Apple process OK, continues on. Monitor goes off, and there is a small amount of disc activity; no joy here.
I'm guessing that the internal graphics won't work under Mojave.
I'm also thinking that my RX580 has died. I guess that would make sense given the white screens I saw.
At this point the MB seems to be OK. I was able to boot Win10 without the RX580. It would reboot with it installed.
So, replace the RX580 with the same thing or is there a better option now?
Thanks for your opinions.