It's not a myth, or a hard fact, but rather a general guideline. We all know there are good and bad cores on a block-by-block basis. Your odds of getting a thick wall are better in an early block than any other years.
As for weight, it's a mathematical conclusion....before I bored my block, I calculated it would lose 10lbs.. After boring, I weighed it and it had lost 10lbs. It's just the weight of cast iron....
