Heater core hole

[latzoo]

I have a 67 Camaro with a LS1 and have seen pics of smoothed firewalls. Without the heater core where does the heat come from with a smoothed firewall? I realize this could be a silly question nut do have the welding bug of late..

[Tiger]

normally people put an underdash heater/AC from an aftermarket supplier in their cars.
I'm looking towards Vintage air for my chevelle but there are others around.