I imagine it to tunnel under the cowl to some extent, or at least be trimmed along the topside so it wouldn't protrude too far. The original cowl vent would be filled so I think there would be a fair enough amount of space to use.

I saw a mid '50s Chevy pickup with a dash similar to the '66 New Yorker style in a magazine a while back - it looked awesome. It is probably way beyond my skill level, too, but I'd like to keep the idea in my mind for when I would actually start on the truck.