Your ranks are not that bad. Just don't forget that the HSC itself is a whopping 50% of your final mark, so if you ace those exams, it is very likely you will get a UAI in the 80-90 range.
If I were you I would not worry about UAIs, marks, school rankings or anything else like that - just focus on studying and doing your best come exam time, that's all you can do. If you can't get into the course you want at the end, you can still transfer!
Good luck