i'd return the 3500s for 2700s and when it comes to time to flower, throw a couple of your 6400s in with the 2700s.
while you're returning the 3500, go for 42/46w 2700s - you really want as much light as possible, especially during flowering.
ideally you'd want 4 of the 42w (or 46) 6400s when...