Going off of the measurements of my B-6, which is loose on me these days, and the measurements of the B-3 on History Preservation (weird that the Eastman site doesn't have such a chart), I'm fairly confident a size 44 would fit just about right, given the thickness of the shearling. It would be awesome to try them on and compare, but considering I'm in the US, it pretty much is impossible. There is an online store selling a size 44, but unfortunately, they don't have a 46 I could exchange for in the event that ends up being too small.
What is your actual chest measurement?
I have to say I'm struggling to understand how 26 1/4" pit to pit is too big for a 48" chest. That's 4.5 inches over chest size on a heavy sheepskin jacket...that's totally normal surely, possibly even on the slim side. These weren't worn over T-shirts either!