Blown head gasket, likely cause????

we've got a 340/418 stroker motor. Which is still pretty fresh. but it's blown a head gasket. In spectacular fashion!
I've not seen a gesket pop this severely before. So am looking for some insight into why it did this. motor has brand new RHS heads from Brian, and the decks were machined when the motor was built. (<1000 miles ago) As well as it's never been cooked or over heated.

any suggestions on Why?