It is possible to get into Mac social science with a 77 average, however I would aim for about an 82 to have more of a guarantee. My friend last year got into social science with an 81.
In first year you will have to get higher marks, as PNB is a competitive program. I don't mean to scare you away, you will be fine if you take courses that you enjoy and will be motivated to work in first year.
Also, be aware that you must take Calculus in first year university. If you haven't taken it in grade 12 you can take an intro to calc course (similar as grade 12) first term and calc second term! In second year you won't need calculus for a BA, you will need to take Stats.
Hope this helps,