It must be a 3 piece longbow to be cut so deep.
Here's the thing... 500's are the right spine for that if you are drawing 28"s or more. Being cut so deep will allow you to move the side plate out toward the arrow if the spine ends up on the weak side... it won't.
400's, generally are better suited for bows above 53 actually drawn lbs.
If your bow is truly cut 3/16" past center, you could likely shoot a 400, but because of the energy put forth by the bow, but because it is closer to center and won't have to flex as much. However, if that arrow is too stiff, the only recourse is to weigh down the point even more and likely leave it full length.
good luck, BigJIm