Hi .. I think arching into the upper back is more Warrior I. With Warrior II it's being aligned on the sideways plane having the ankles, knees lined up, the arms lined up over the legs... and the spine rising up from the pelvis.
For alignment imagine you're doing the pose on a balance beam - arching at the expense of balance will tip you over. The Warrior has balance, poise and inner strength. Maybe if you imagine your chakras all lined up in a plumb line from your crown down through the whole spine into the floor, you won't need to arch to feel the power. Try balancing the skull on top of the tailbone with the whole spine alive and breathing. Feel the chakras radiating light and awareness and you may feel exhilarated - there are other poses where you can open your chest like an eagle. Not eagle pose though
