Thank you to all for the feedback.
@Mac Max: You are correct on the leak down test but unfortunately I'm too late as the heads are now off. Now that they are off I will re-seat the valves anyway.
@Stephan: Thats what I was looking for. If I de-rate the Std/Max/Min psi values for altitude then I am not way out. In fact between Std and Min. for #4.
This water jacket pipe seems very under-engineered. What is it, plastic, fibre glass? I am going to investigate getting something turned from the appropriate grade aluminium. Maybe with an extraction screw. Any comments on this?