340 or 360?

But my point being is in 79 when the X block came out the 340 was long gone. So it was my understanding that the X block was based off the 79 318 block that was reinforce with thicker decks, thicker webbing, thicker bores, and higher nickel cast iron casting. That is something that been stuck in my head from something I read about the block back in the early 80's.