Diego,
I'll give you my best-guess answers to each of the two questions you posed.
1. The Chevelle SS396 was a new offering for 1966 (ignoring the very limited production Z16 for 1965). It came standard with 325 HP. Chevy wanted to steer performance-minded Chevelle buyers into the SS396 model, and if they had offered the 350 HP L79 in the Malibu and the 300 models, it could have stolen some of the SS 396's thunder.
2. In 1967, the Camaro was a new offering for a small, sporty car. Chevy wanted to steer buyers into the SS350 Camaro so did not offer the higher performance engine in the Chevy II. There were so few L79 Chevy II's made late in the 67 model year that you might as well say it was unavailable all year.
Those are my best guesses, and I'm stickin' to 'em
