I used just under 200LB for reps of 8 on the good morning and it really didn't feel 'right' so I didn't try to go any go heavier, I've seen those West-side videos of the guys using 400LB for reps on the seated good morning and I wonder how the fuck they do it without fucking themselves up.
I would err on the side of caution with good mornings, and would never go heavier then a triple.
I haven't found them to be particularly helpful to my training and would experiment with using exercise variations like various squats including safety bar squats, various dead lifts, reverse hypers and hyper extensions. I find kettle bell swings are great for increasing posterior chain strength.
I think you need to be at a certain level for good mornings to benefit you, either that or have a particular type of body structure, either way they don't benefit me personally and as usual there is more than one way to skin a cat.