The 6400 does not work with 7.5.3, with or without an enabler. You need 7.5.5 with the enabler or you need to go out and find a copy of 7.6.1. I'm not sure if Apple's 7.5.5 update would have the enabler with it.

7.6.1 may be your best bet, check not only eBay but also vendors like WeLoveMacs.com, many of them tend to stock NOS versions of the OS.