Fishbum: Take alook at the Placencia website for fishing opportunities in the south. Generally more permit in southern Belize, more tarpon around AC. Also take alook at www.belizeflatsfishing.com.
They have opportunities all over the country, and a brouchure that'll make any flyrodder drool. Later spring fishing seems to produce larger fish, esp. tarpon, but numbers are great any of those months. If you decide on AC, repost and I'll offer at least my take on places to stay and guides for flyfolks.