Hey guys, I got a question about the valvetrain. I recently added a adj. vac advance. I got everything hooked back up plug wires...etc. when I tried to turn over the engine and fire it- she wouldn't fire. I let it sit for a few minutes, checked the connections and tried to fire it again. While cranking the engine over it made a huge backfire and then started and seemed to run allright. I then took off my drivers side valve cover and noticed that my #7 cylinder roller rockers were off and one of the pushrods were broke. I took off my intake and located both the pieces of the broken pushrod and found the other one in the lifter valley as well. Also both lifters came out of their bores too. I had some new ones so I just replaced them. My questions are below.

1) what could have caused my pushrod to break?
2) what else should I check before reinstalling my intake manifold?
3) was the loud backfire associated with the broken pushrod?
4) do hardened pusrods break like mine did...In half?
5) are heat treated pushrods the same as hardened?

sorry so many ???'s but I don't have anyone to help me out.