You want about 0.030" between the chamber and fire ring. ATM I do not have a single Pontiac head to compare to your 0.256" measurement. Hard to judge by the photo. The photo shows a lot of oil is reaching the chambers, though.
On the chamfer, I should have said notches (arrows below), not the small taper right at the top of the bores to help the pistons and rings to fit. The notches I was asking about are there to help flow. If that's also what you meant, disregard this.