Adam, I called Herreshoff designs and they didn't have too much to offer regarding the engine air flow. Though they designed the basic hull for the 18 footers, they left the detail work to the builders (in this case, Nowak and Williams). But it was great to talk with them none-the-less. They themselves use an 18 ft launch.
I've decided that if I have engine breathing problems, I'll just install a vented face panel on the motor well. We'll see.