I, too, have watched this episode several times and wondered where the airport was located. Since the episode was aired in 1968, and given the way the greater Los Angeles area has grown in the last 40 years, I thought the airport might no longer exist since it was so small.
After reading the initial question and the follow up comments, it appears you were correct about the airport being the one near Pacoima.
I am watching the episode right now and saw the smoke stacks in the background. I went on Google Maps, found the area, zoomed in a bit, and switched over to the satellite view.
The smoke stacks can be clearly seen. They are located east of the airport.