>>410548609
With lazy headswaps you'll usually just fill in the underside of the head, and have the neck end in a point so you can just "stick" any old head on it.
If you want a perfect seam though, extrude the edges along the back and match them up to the head, then add a triangle where they meet the non-extruded section.
If these were the same mesh ( you should keep your head mesh separate ) you could just select he edges on both sides and do Bridge Edge Loops