You could sand the top with 320 if your going to prime the entire top. Finish sanding in 400 after the Primer has cured. It's a good idea to repair all cracks before you prime, hopefully the one in the footwell is the last one, if not, the use of guide coat over the Primer will let you know, you could spot repair any other cracks as you see fit. Paint will actually accentuate any cracks...so it's a good idea to fill ever and repair everything you can before paint.
Use a poly coated masking paper available at any auto body supply store. Don't use cheap Home Depot masking tape, get proper automotive tape. It'll save you time, money and aggravation in the long run.