I think we tried this same stunt with a Rev D iMac.
The problem is HP's drivers running under OS 9 (or 8.5 or 8.6 for that matter). They haven't bothered to update them for that old a scanner. We finally ended up selling that print shop an Agfa SnapScan Touch, which has worked darn near flawlessly.
My suggestion: junk your scanjet and look over this forum for scanner suggestions. I'd recommend an Agfa SnapScan Touch or 1212u, but that's my opinion. YMMV.