The Crown has a either a DD 6-71 or a Cummins small cam 743.
The Gillig is one of hte last series before the Phantom so it is an '80 or '81. The double sash windows are larger than any other school bus windows. With the standard 78" headroom it would make a great candidate for a conversion. I am not sure what transmission it has by the description. It may have an air shifted Allison.
The BB would be a good choice, particularly if it has the 12" windows and high headroom option.