[4x4Poet]

Factory A/C systems have a fresh air feature to draw in air from the cowl so occupants don't breath recirculated in cab air. In all I've seen web researching and reading mag install articles, aftermarket systems (Old Air, Vintage Air, Classic Auto Air) have only an intake under the dash. This is unhealthy, IMO. I think the aftermarket A/C companies do this to save costs. Having a fresh air duct and flap costs $$. Maybe aftermarket companies also think a cleaner firewall is more appealing to restorers.

Consider pricing a professional refurbishment of the factory A/C system. No A/C system, no matter how old, will leak R12 if all the components are serviceable. You could simply have a leaking component like a hose or condensor, etc. A shop that knows their stuff would first put a vacuum on the system to find the leak and go from there.