Dad's shirt pattern

I am going to cut it and make it out of muslin and bring it over this weekend or early next week. I can see how (if I am guessing correctly on the issue) I can add width to where the sholder and neck intersect so the shirt can cover that, lay flat and not pull up. There is lots of extra "dad" up there and I think that (in combo with his posture issues) that mess with the fit.