Hard to say on where to open up the chamber.

It depends on throat angles, valve job. Weather air swirls and follows air paths into the chamber or is very turbulent going into the chamber.
As I said before, not good at explaining how this all works, I do know how it is supposed to flow into the chamber though

Sometimes, a valve will look like it is pinched off & is so close to the side of the chamber, you relieve the chamber wall & it does not really help, well that is because that is not the way the path of air/fuel is entering the chamber.

From what the ported guy did to the other cyl head chamber walls, it looks like a lot of airflow enters the back side (close to the spark plug hole) of the chamber. That is how my cyl head picked up a lot of CFM, but mine is aftermarket.

If that guy knows how to port those Pontiac heads & is rebuttable I would just try & copy his porting techniques.
Or were you going to have him port it?

As far as compression goes, I would shoot for 8.5-9.0 compression ratio.

Again, not too sure you should go as big on the ports as the gasket is? Unless your flange is made smaller than the intake/exhaust gasket.
Bigger is not always better.

12 port SDS EFI