Here's about seatbelts: http://www.camaros.org/seatbelt.shtml Shoulder belts became standard in Camaro coupes in Jan 68. Shoulder belts were available in convertibles. Several pics of them in my article.
I only see a few cars with 742's on PHS docs and I find them both on converts and coupes, after Jan 68. And there's converts and coupes built the same time without the code.
