I have always considered Conan #1 to be the first Bronze book for the following reasons:
1. It was a completely new concept in comics. (OK, Kubert did "Crom the Barbarian" and "Tor" back in the 50's, but neither really caught on).
2. It was immensely successful, spawning many spinoffs (Savage Sword of Conan, King Conan, Red Sonja, King Kull) and imitators (Arak, Beowulf, Tor (again), Wulf the Barbarian, etc. Even Cerebus was originally intended as a Conan spoof!
3. It was published in 1970, the beginning of the new decade, a natural delimiter.
4. It was the first book that was widely purchased in bulk by speculators.
5. It received tremendous coverage in the media. There really was Conan mania in the early 70's! It brought a lot of attention to the comics industry.
There are good arguments for Green Lantern #76 as well:
1. It came out 6 months before Conan #1
2. It changed the way comics were written by addressing an important social issue of the day.
I'm sorry, but I don't see any merit in the arguments that Giant Size X-men #1 is the first bronze book when it was published 5 years after GL #76 and Conan #1. The most important bronze book? Maybe. But certainly not the first.
The only other major event that I can think of that pre-dates GL #76 would be the Marvel expansion of 1968 (Tales of Suspense splits into two titles - Captain America and Iron Man, Tales to Astonish became Hulk and Sub-mariner, etc). But I have a hard time saying Iron Man #1 is a bronze age book!