I know here in Virginia stereo sound was rare for most theatres in the mid-70's. The place I saw KK 76 was a tiny little triplex which was equipped for both four track/35mm and six track/70mm in Churchland, a suburb of Portsmouth. They were the first in VA, as far as I know, to install Dolby...
KING KONG 1976 did have some release prints with four track magnetic stereo sound. I saw one back in the original release. So I am not sure we can say mono sound was its primary mix.