I was very pleased with my find. Now lets look at some horrible pictures of muslin's.
I used the Negroni shirt pattern from Colette/Walden. Muslin 1 is a straight size XL. The result isn't too bad. I pushed shoulder seam back a cm, raised the arm hole a lot, gave it a little extra room in the hips, and shaved off a little off the bottom of the upper back piece. The Brother has rounded back shoulders so there was extra room there and I really wanted that seam to look horizontal.
Muslin 2 looks much better. We were happy enough with it to cut out the good one. This took longer than I had hoped. I don't know what I did when I prewashed it but it was really off grain when I laid it all out. I cursed and swore and wrestled with it for a few hours. Some googling later I found Madalynne's article about grain perfection. I sewed the whole thing together selvage to selvage, ripped end to ripped end and washed it again. It worked! mostly. The ends were bang on grain, some parts in the middle were still a bit skewiff. I guess it works better when the length isn't 5 meters. The buttonholes were the real pain. My machine from the 80's has step by step buttonhole, but I don't really like it. I have some vintage Singer buttonholers that I really like and have had good results with in the past. But last night it just could not make a good buttonhole consistently. Never let the machine know you are in a rush!