If you look as some cars with factory A/C, you'll see for yourself that the blower motor is larger on a A/C car. So the hole on the firewall is also larger compared to a non A/C car. You will have to cut another hole in the firewall to let the evaporator tubes out in the engine compartment. If you go the aftermarket way, (Classic auto air, for exemple), everything goes out to the engine compartment through the original blower hole, so no modification needs to be done to the firewall, but, (in my opinion), it looks weird under the hood without the blower, (the Classic air kit hides the blower inside the car). Hope that helps.
Robert