Okay, so I quickly sewed together the muslin this afternoon...and had a bit of a disappointment. The fit through the shoulders was great, so I'm glad that I added all that width at the shoulders. However, I still had some binding when I reached my arms forward. I double checked on the "model garment", and no binding at all there. I did notice when I was sewing the muslin together that the grain on the sleeves was all wrong (my fault, I fudged on the grain line and paid the price). With the grain all wrong, the armsceye was very crooked, the seams did not match at the underarm. I think that contributed to the problem. I went back to the pattern and checked it against the pattern I made from the model top...and I could use another inch across the back. I knew this when I was adjusting the Simplicity pattern, but I decided against it, since I wanted to add to the shoulders and the upper back just by making one adjustment. Now I know that I will need to make two adjustments. Live and learn.
I just finished taking off all of the extra vellum paper that I added in order to make the original shoulder/upper back adjustment. I think I'll trace off a copy of the pattern once I return it to its original state and start playing with the copy. The adjustments I'm thinking of using would just destroy the original...and I've learnt not to make any permanent changes to the original pattern, just in case! And I'm usually too cheap to go out and buy another copy of the pattern, even on sale.
Anyway, I will work on that tomorrow. I really hope I can solve this problem...I would be able to sew and wear all these patterns for woven tops that I have in the pattern stash!