I'm not doubting an increase from a head alone.
I'm saying the right cam selection will produce more power up to a point over just a head change on a stock cammed block.
Here's a 4bbl xe262h cam and headers on a stock 318 long block makes 282hp @ 5000 rpm
How much more hp could you squeeze out of the 318 heads with what cam do you think?
I'd say at least 300 hp but probably somewhere between 300-325 hp without adding more cr.