RE: Good mornings
good mornings are a great exercise but very taxing on the body and CNS. If you are using them to assit Squats and deads then i would suggest you use a wide stance. I normally have my feet as wide as the power rack.
Have the bar as low on your back as possible. When you begin your legs are just about straight, as you bow forward slightly bend the legs (1/3 rd of the way of a squat). Make sure you keep your chest puffed out, your abs pushed out, your head right back, an arch in the lower back and keep your midsection tight. When your upper body reaches parallel with the ground thrust yourself back up placing the focus of the lift on your hip flexors.
Good luck bro.