Another head inquiry...

ANY big block factory iron head can be made to deliver around 600 HP. It depends on what you do to the heads. Some of the factory iron heads when ported, like the 906, can deliver 750+ HP.

Give two mechanics the same box of parts and you will get two different results. It is dependant upon each individual's skill level assembling what they have to work with.

Prepare the parts you already have properly, like the 516 heads, and you will surprise a multitude of people. Maybe even yourself.