You don't need to use a single board. I usually use a couple of boards which I glue together. You have to use a plane to make the pieces fit precisely, that there is no space in between. Then I glue the several pieces together using painters tape, (which is elastic) on both sides. After the glue has dried, I use a plane and make the whole surface equally thick.
The body block I have made from walnut, the dark pieces of the body are mohagany and the light parts are maple.
The bottom I have made from a tropical piece of wood, which I got from a piano builder.