If your bass has a gloss or painted finish then it will be difficult to expand the pickup cavities without chipping the finish along the edges. Normally this kind of work is done with a router or dremel using a template so you get nice straight lines and properly radiused corners. You don't get any second chances if you make a mistake. You'll need to remove just over 1/8" from each side which doesn't sound like much but it will be very noticeable if you're off, especially with the radius of the corners.
So unless you have the tools and the chops I would find out how much your local luthier/tech would charge to do it. If you're not obsessed with cosmetic perfection of the bass then it should be easy enough to remove such a small amount of material.
