Why do Flathead V8 Blocks Crack?

[flatheadmurre]

I have thought about the idea of heat threating a block to releave the stress and then throwing it in the dark hiding to sit and age....
Thats what you do with a new cast iron piece but should it add something to an old block and how much ?
Have any of you guys some of the valvepocket sleeves left ?
I still have the kit but im not shure if i have any sleeves.