I still think this is the easiest, if not the most practical place to get cool air. I don't think the Z/28 air cleaner will fit around the blower fan, and heater core. There would be a enough room to cut a 4" hole in the firewall, then rout a 4" pipe into the cowl area. You could probably adapt one of the fresh air kits that uses the cone shape air cleaners at the end of the pipe. Placing the filter in the cowl section. This would give the most under hood clearance, because the filter wouldn't be on top of the carb.
(EDIT) I take that back. There is room for the Z/28 Cowl style air cleaner, and not enough room for a cone style cleaner in the cowl area.