Thanks for pointing me toward ASD. Obtained a copy and after some effort was able to get the G5 to start up enough to load ASD and ran it far enough to see that there's no need to calibrate the fans, at least at this point (no surprise since the CPU's haven't been removed from the MB). However, when I try to run the hardware tests the system freezes solid shortly after test start and the fans slowly increase to max speed.

There were other problems when I was able to boot from the HD, including KP's and other freezes. If I can't find a replacement MB for a reasonable price it will probably be time to part it out. Sigh!