You are not going to be able to fillet the edges of the body, it's too complex for autocad. You probably get an error message, right? That's ok, though....it's only 2.5....
For the neck try this (if you haven't already):
Draw the profile of the neck at one end ofthe fretboard, then the profile of the neck at the other end. Then, do a loft or sweep. This way, you get the width of the fretboard, the thickness of the neck and the contoured shape of the back.
Here is a crude example in Inventor. AutoCAD should have a similar feature....it's been a while since I used ACAD