Another small block turbo early valiant

How are you logging boost pressure? I would verify that whatever sensor is measuring 23.5 psi is accurate for starters. Matts440 is correct, you need to give more priority to exhaust flow through the wastegate and even that might not be enough. You may also need a larger gate. And at that boost pressure I would look at the headgaskets first hopefully your problem is there and not crack in a cylinder or head. Good luck. Keep us updated.