It's not a simple process to understood if you are not following the BootROM thread, but i'll extract the 3rd and 4th streams of the NVRAM and the full LBSN/MLB sector from your BootROM dump. I'll validate both, correct/upgrade if needed, then inject the two streams in the empty NVRAM extracted from 10.14.4 generic BootROM.
Next step is to inject the recreated NVRAM and the original MLB/LBSN sector in the current generic MP51.fd release, 141.0.0.0.0.
After all done, you will have intermediate files to recreate any MP51.fd version, from MP51.007F.B03 to 142.0.0.0.0, tailored for your Mac Pro and a functional BootROM image of 141.0.0.0.0 already injected ready to flash with ROMTool.
Instructions on how to do this will be sent by PM.