Thank you! I guess the fit should be good if I go with the 36 size where pit to pit is 19"With a 41" chest and a size 42, I have room to layer a sweater. With a size 40, it is a trim fit (but not tight) with no room to layer. I think the same would hold true if I had a 42" chest.
The p2p of a size 42 peacoat is 22 1/4" +-. The p2p of a size 40 peacoat is 21 1/4" +-. These are sizes for the US Navy peacoat. Not sure what the civilian copies are. They probably vary quite a bit.