The most important part of the application for you in is the SOP. Ask yourself: why do you want to study Biostatistics? This answer is what will set you apart from other international students whom you are competing for the ultra limited admissions spots. I'm sure you'll ace the GRE and you really don't need anymore coursework, you need to figure out a way to set yourself apart. Seriously, a lot of people undervalue the importance of the SOP but I guarantee that my SOP is the reason I was accepted to all 8 schools I applied to, including 2 schools on your list.
Also, consider applying to more schools. UNC Chapel Hill, University of Minnesota, and University of Michigan are great schools. Additionally, I don't think Stanford has a Biostatistics program and reconsider applying to a school in the UC system - budget cuts are unpredictable in the west these days.