I need your advice

Do I have a chance at good US unis if i have a 70% in 12th boards? I will be a gap year student. I have good Extracurriculars related to my major and also not related to it. My test scores (ACT 34, SAT sub tests- 800 on 3). My intended major is CS. Would it be better if I sit for the improvement exams in July to increase my board %?