Big problem is that you have to clear customs. I don't know which passport you will use. But US customs is never in a hurry. If you work on it you should make it. Make sure you get all your boarding Passes when you check in. The United terminal is the next terminal to the international terminal.
It would be a tight fit because if your Qantas flight delays u might miss your United flight. And u would need to check your flight # for the terminal info
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