I welded in new panels (allowing a ½” opening at the bottom of the firewall/air box -- each side) for the outside air ( as well as rain, wash water, snow, etc) to continue the flow in to and out of the openings under the windshield stainless. These opening were used to force outside air into the heaters (one on each side of the interior). Blocking these by welding them shut would result in rust, eventually – when -- not if -- they filled with water and debris.