Another thing to think about, 350s started in the Camaro in 1967, then for a lot more vehicles in 1968. A 350 blew up easier than a 327 or a 283 (shorter stroke) and there were fewer forged cranks in these engines. With more and more Super Sports out there and a general increase in racing, I think failures were getting to be more common. More cars were being abused.
In 1969 there had to be service assemblies for the 283, 302, 307, 327, 350, 396 and 427. Think about that.