I have the RMC J100 in a 42
I'm 5'9 177lbs with a 42" chest and I only wear the jacket unzipped with a tee, and it fits a thick hoodie under just fine. You gotta allow at least 3 inches in the chest to breath.
I went to the shop vancouver today. I saw an iron heart type III IHK 64 that fit me well but not sure thats quite the style I was looking for. I also saw a Y2 leather EB 140 in black (almost brownish) 1.8-2mm thick Leather again similar style to the iron heart and a lot of $$$. I still love the RMC J100 clean simple lines. The fit above was super helpful thank you as we are nearly identical I am 39 years old turning 40 which this will be my gift 6’2 and 185lbs with 42” chest. This post really helped me and I think the 42 in the J100 would be the way to go! Thanks again.