My 54 Dodge originally had a hole (about 3" diameter) in the firewall near the heater for fresh air. Under the hood was a short tube with a butterfly valve that was at one time cable operated. I suspect originally it had an expanding paper/wire tube that connected it to the grill area near the radiator. Even a hole connected to the Classic Air unit with duct work through the firewall and going towards the fender would help a lot. Any fresh air would be better then none.