8 hours a day really isn't that stressful. Think about it, if you're awake for 16 hours a day, then all you have to do is study one hour on, then have one hour off. Rinse repeat. Simple, no?
I think 67 is quite achievable, your ranks aren't rock bottom, more just below average. You're kinda relying on your cohort to do well in the externals at the moment, just do as well as possible.
One hour revision for one subject
One hour break
One hour revision for a different subject
One hour break
Rinse repeat for 14 hours a day until the HSC.
+1, but its harder than it sounds.
There's literally no point in doing more than ten units. If you're an organised person, you shouldn't need backups. Though I believe its important to do as many units as possible in year 11 to find out which combination of 10 units will best suit you.
If you keep it up you're looking at 98+
I'm not 100% sure how standard english will scale for you, but with a high rank like that, I doubt it will drag you down too much.