Is it possible without a major workaround to load OS9 on a 6400? Each time I try the machine tells me that the software can not be loaded onto this machine.

I've run 9.2.2 on my 6400 using the application bundled with XPostFacto. OS9Helper or something I think it was called. 9.1, as many have pointed out already, should install from the CD onto a 6400.
Andrew