For 1966 the Beaumonts came with Chevelle seat upholstery. material and patterns. They did not offer the full range of colours allegedly, although I have seen both a '66 Chevelle Malibu SS & '66 Beaumont Sport Deluxe with turquoise bucket seat interior (supposedly not available in Canada). The Custom & Sport Deluxe used Pontiac LeMans door panels together with Chevelle seats.

As a point of reference: https://chevellestuff.net/1966/interiors/index.htm

There was the base Beaumont with the Chevelle 300 interior, or there was the Beaumont Custom with cloth bench seats (A51 Sport Deluxe had vinyl buckets). There was no equivalent to the 300 Deluxe in 1966/67 (there was in '65 & '68).

edit: I should have mentioned the Beaumont Custom & Sport Deluxe had different interior codes than the Chevelle equivalent because of content differences like  the door panels. That was also generally the case with full-size Pontiacs like the 2+2.