I'd look at it this way - if you book it, you have a chance of getting the 763. If you miss, it's not worse then having booked the 762.

I'd take the chance.

BTW - if you are late out of SYD by any significant margin of time, UA will rebook you automatically to protect you on your connection. Upon arrival in SFO and clearing customs, you may want to make a cellular call to your reservations number to see if you've been rebooked. However, do not delay getting through the process by making this call first - it will decrease the odds of you making whatever rebooking services were done on your behalf if you are stuck in a crowd in customs.