I'll look again when I finish the downloads, I browsed Mac Garden for the 2005/PCIe (which if I searched correctly should be what the 7,3 is) install disks and couldn't find them.
10.5 would be a slam dunk on that machine, but wouldn't run classic.
If you can swap that machine, or find something else to run whatever you need in Classic, that's what I'd do -- 10.5 on that G5 will run well and will make much better use of that hardware than any Classic app will.
This is still, I realize, not what you asked, but any software that runs well on a G4 on OS 9 will probably run better as a newer version on a G5, especially those last-gen ones, especially-2 if it happens to be a Quad.