Nice looking hoss you got there Tuna! He's beastly for sure!
As for head shapes, I'm convinced that there is just a specutrum of possible shapes. I suspect that Tuna's and Sashimi's GATF are at opposite ends of the spectrum. There is nothing in the literature that would indicate that Hydrocynus goliath is sexually dimorphic. This seems to just be a luck of the draw type deal to determine what type of head you get on your fish.