Expert Flyer will allow you to create a free seat alert if a taken seat becomes free. Also, if you have QF FF status, you can enter your number into your profile and you'll see what seats are available to you (as a WP I have most seats bar the exits and the front few rows in Economy).
You can select your seats at T-80 I believe (not had to do it as I do have status). I'm sure someone else here can advise. The best bet is to login as near to that time as possible and select from whatever seats you can see.