I don't think anyone chambers for the 375 Win anymore. My 94 Big-Bore dates to the early 1980's and predates the angled eject so I can't even put an optic on it. Not that I likely would as I've never cared for the look of an optic on a Winchester or Marlin lever gun. I'd probably opt for a .38-55 if building something on a modern break-action single shot as there's some pretty stout +P load data out there for it now. Basically the same cartridge, but the .375 Win is just a hair shorter to ensure it couldn't be chambered in an older 38-55 rifle due to it's higher pressure. IIRC originally the 38-55 was rated at 32K PSI with black powder and I think the +P loads are around 38k PSI while the 375 Win is 52K PSI.