They make a cover to convert an AC car to a non-AC car. Kind of expensive for the part, but it allows you to do it without welding up the hole.
Camaro custom Air Conditioning cut-out heater delete cover. Cover is for factory equipped cars with Air Conditioning. Eliminates the factory firewall evaporator box, offering a clean under hood look while sealing the firewall, allowing a completely functional heater system. Your existing stock AC underdash heater box, heater core, defroster and control are retained for fully functional heater and defroster. OE quality die-molded fiberglass with heavy pre-finished black gel-coat. Made in the USA!