If you're not planning on installing any PCI cards, you could swap out the heatsink/fan combo for a heatsink (sans Peltier junction) from a PPC 601-based Mac like a 6100, or a 7200.

It'll be much taller than the 6500's original heatsink, which is why you couldn't use PCI cards. However, it should snap right into place without bothering anything on the 6500's logic board and keep everything cool without a fan. I've replaced the flimsy tiny stamped aluminium heatsinks on 603-based machines (4400 and 52/6200) with the larger ones from 601s in several machines with no issues, other than blocking some expansion options in more compact machines.