Air conditioning hole

I just yanked the engine from my 72 and I'm getting ready to weld up the stock air conditioning hole on the firewall because I'm going to go with an aftermarket system. Does an aftermarket system utilize the stock fan or can I weld that hole up too? I've noticed some people weld up the stock fan hole as seen in the picture below. If you weld up the stock hole, how do you get air from outside?

http://www.theosbornfamily.com/images/nova/firewall.jpg

Thanks in advance.