OP checking in again. Sorry for any confusion. As Swingman correctly
deduced, the sides are frame-and-panel, and the front is a frame with
doors. So wood movement shouldn't be an issue for the main body of the
cabinet. I guess the reason I posted the question was the fact that
this is a relatively narrow top, and I wasn't sure if there would be
enough potential movement over 13" to make it worth worrying about.
The consensus is that there is. It will be easy to attach the top
using cleats or clips, as suggested. Thanks for the advice, everyone!