It's worth a try ... however, it does not sound as efficient. The entire configuration was designed to maximize convection, i.e. the hottest components located near the top to draw cool air up. Forcing air down through the Mac would actually send hot air down into the case.

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I added a fan to my iMac 350MHz and it was blowing on the logic board(the other side where the HD and disk drive are located) and it worked just fine. I added an ATX drive cable expander to the existing ATX cable inside and it worked just fine. Not sure if the older macs have the regular ATX connections but its worth a try.