I'm a little concerned with the 5' opening and no vertical brace and only a single 2x4 as the span. Anything greater than 4' and IMO you'd either need to use heavier lumber (i.e. 2x6) or sister that span and add an additional 2x4. Of course your other option could be to shrink that opening by about 6" on both sides with vertical 2x4 supports and i'd be able to sleep better at night with a 4' opening. Check out this thread if you haven't already. It has some really good info on building a DIY stand:
http://archive.reefcentral.com/forum...5&pagenumber=1
Are you facing the outside of the stand with plywood? If so, and if done right, the plywood facing can also act as an additional header support and might make it ok. However, i'm always extra cautious when it comes to stand building.