Corner gussets won't do much, the big problem is the door openings at the jambs on top!!! They need very good support, if the jambs move and you don't catch it and straighten them, you'll have a van full of grief forever!!! Don't know that a halo would do much, good bracing with a few X's for added strength would be better. Bracing is more to do with the correct structure then anything else.