It's just my opinion, and I'm not a tailor, but I do not think that you should widen the shoulders at all. The shoulder seam should sit on the top of the edge of the shoulder, so to speak. If you widen the shoulders, the seams will start to fall off your shoulders down your arms - just like so many ready-to-wear shirts that I see around the place. In fact (and this might be controversial, so I'd welcome other comments), I think that you could actually benefit from narrowing the shoulders a little, so as to make the shoulder seams more "upright". I certainly don't think that you need to add anything to the back as there seems to be quite a lot of excess fabric there already.