Unfortunately it's going to be really, really hard to guarantee anything on this route. Delta does tend to be pretty good, I find, about opening upgrade space within a month of departure or so -- I've seen many of my own RUC and GUC requests (on other less high-demand routes) clear in that timeframe, and I think I've seen occasional reports of GUCs clearing on LAX-SYD ~2 weeks out. So in theory if you could wait to book your ticket until closer to departure, you might find some options that are confirmable at booking ... but your cash coach ticket will probably cost a lot more by then. And it's still not a guarantee, especially on that route.