If it were mine, I would build or buy block-off plates for both the heater core holes and blower motor hole. Attach with screws using existing holes and factory seals, and also paint to match engine compartment or use the natural finish of the metal. Easy to undo if you decide to run heater at a later time. Here's one you could buy. or copy, for heater motor:
http://chucker54.stores.yahoo.net/maraheblblof.html
