350 spit out cam, any ideas why?

[Rifter_Chevy]

So my engine went boom a few weeks ago and im just throwing around possible causes in my head and want to hear options, doesnt really matter but should be some fun discussion.

It was a 350(mostly), it was not taken care of by previous owner, when i bought it it didnt even have a oil dipstick and had less than 3 liters of oil in it and was leaking oil from every possible spot it could leak oil from so i guess it ran in a state of oil starvation for most of its recent life till i got a hold of it. I drove it for a year till it went boom.

It had a 305 head on one side and a 350 head on the other(didnt notice this till i pulled it after it died), was a 350 block 4 bolt main.

I was driving on the highway and noticed a lack of power, like a huge lack of power i couldnt maintain 40mph up this hill i can usually do 65mph on without even trying. As i started to pull over engine stalled and locked up solid had to put it in neutral to roll off the highway. Puked out a very small amount of oil that was pitch black and water like(oil level/condidtion was good as of 3 days prior when i checked it last). There was no oil in it and was blowing plumes of white smoke out of the valve cover breaters and the oil dipstick tube when i pulled it out.

So I pull the engine and fine out that the cam snapped in half between the 7,8 and 5,6 cylinders. And then spit the cam out of the block into the flywheel really messing up the tourque converter bolts and making them really fun to remove.

The rocker studs on 7 and 8 were snapped off, 3 pushrods bent and one gone, never found it. One lifter was also gone and i never found it either. TONS of metal shavings in intake valley, probably the missing lifter and pushrod remains but the largest piece of metal shaving was the size of a uncooked grain of rice.

So what do you guys think would cause a cam to blow out the back of a block?