I thought I had about an hour yesterday to work on the muslin, it ended up being less than that, but I did make progress. Thursday is knitting night, and even a muslin in process wasn't going to keep me away.
I only managed to put together half of the jacket. Even thought the back is cut in one piece on the pattern, I cut the muslin as two pieces just for ease in fitting. It is rare that I cut a back piece on the fold anyway, even though I will cut the final back in the final version in one piece. Usually there is just enough difference in the right and left that I need to work from a full pattern piece.
This is what I learned so far:
The 42 looks like it is fine through the shoulder and through the back, at least the half back, that is already made. I need to release the seam a little bit at the hips in back, but so far there are no other major back adjustments. When I have a full garment I may see other changes, especially at the sleeves, but I have the right size pattern.
I definitely need to do a FBA, but I was expecting it. Otherwise, the front seems to fit well through the shoulders and upper chest.
Today I can make and attach the other half back and do the FBA. Once that is done, I can see what other adjustments are needed. I am sure there will be something, but I am feeling my confidence returning.