I took a look at the other shape modification below. The only way I know how to make it without a seam or a noticeable transition, is to make a cutting surface (magenta colored curve) and slice a cut across the entire guitar body.

To do so, extrude the guitar body about a quarter inch taller than you need. So if the final guitar body is to be 1.75" thick, extrude it to 2".

Draw the magenta colored curve. I show it with the control points turned on. Note in the Left View that the top 3 control points are horizontally aligned. (Your alignment tools will also work to align control points if they are selected.

Extrude this curve across the entire guitar body.

Boolean Diff this surface with the guitar body and delete the top portion of the body. Note that the angle of the magenta color curve is important as you don't want to modify the shapes in other areas of the guitar.

Fillet at 0.2" Shape: Const Dist

Ed Ferguson