Building 340 short block

He adds block filler for insurance on a street engine? I'm not sure I'd trust him if he's that concerned. He is right - filler does strengthen - at a cost. That cost is both financial and cooling system efficiency, plus more maintenance and and faster engine wear (the oil gets hotter too). It's entirely up to you but what he's telling you (yes, in terms of piston choice too...) is of an "extreme" nature. He's "seen" too much in terms of destruction and he's planning too much for it. If it were me I'd find another builder. You'll get a better product for less money.
My advice - sonic test the block - and it should cost less. $400 is robbery. Then buy a set of pistons based on use. There is nothing wrong with hypers provided the builder follows directions in terms of piston to wall and ring gaps.