Pusher Fan Advice?

And think about this.
Performance alum heads have bigger ports, with plenty of extra metal for additional porting. The decks are usually 9/16" to 5/8" thick, compared to about 3/8" for iron. Valve spring pockets need to be thicker because alum is not as strong as iron. Likewise with the alum surrounding the head bolt bosses, accessory holes.
All of this intrudes into the coolant passages, reducing the volume available for coolant.