Paul, take a look at this picture of one of my MBR vanities...you can see what I'm talking about on the top of the unit...the wide, horizontal strips to the front and back of the carcass. You can install them in your unit with pocket screws and glue and when you do that, they will make for a very strong combined component with your front and back top stringers. An L-shaped "beam" in a sense. These are, in fact, notched into the intermediate supports which in this vanity are similar to what you will be providing. This further locks things together. The tops for these are also granite...in fact, they templated today according to my GC.