Select just this face of the head of the guitar, and make it a component.
Select everything else, and make that a component too.
Then select all, and reverse faces, to get white faces outside, where they should be.
Delete the rest of the ‘head’ parts, to leave just the component face.
Delete everything in your ‘cutter’ area except this top face:
Open the ‘head’ component for editing, and pushpull it past the neck of the guitar.
Close the head component.
Push pull the top face of the ‘cutter’ part down below the head, and window select round it.
R-click, and select Intersect faces/With model, to create the sloping faces you want for the head component.
Close the main component, open the head component, and select it all.
R-click and Intersect faces/With model again, to create the cuts round the edges of the head.
Trim off the excess in the head. Use a R-L selection, and erase what is selected.
Repeat for the right side.
That should have created faces in the ‘cutter’ block in the profile of the head, but the edges have gaps in the top portion.
But it has done on the other side. Double click that face to select both the face and its edges.
Select that face, and cut to clipboard.
Trim off the rest of the cutter block and open the head for editing.
Edit/Paste in place the face from the clipboard. Double click it to select the face and its edges.
Move/Copy that face to the other side an d reverse it, then ‘stitch’ across the ends from endpoint to endpoint to fill in the missing edges and create the whole surface round the ends of the head.
I’ve only done part of it. But here’s the result
6 string single cut bass build JWM.skp (236.0 KB)
As DaveR suggested earlier, make more components of parts of your guitar, to keep the geometry separate. It makes editing MUCH easier.