small block build ?

The lesser the head, the more built up of other parts you'll need to have done. And a large cam becomes part of the program.

The production iron heads as cast can support 400 hp. The rest of the engine needs to punched up to the extreme side of the build while the heads remain stock, save the valve springs of course.

In order to get 400 HP from a stock production head, the cam must be a bit large and a well balanced and rounded build it is not.

Better flowing heads like a well prep'd set of RHS or Edelbrock heads will make that HP with a little cam, be super streetable that even ol'cranky grand-ma will drive it without complaint.

You already have enuff info from this thread, save up the money to get what is needed.