If you are really on a budget. You would be surprised how well a coat of wax protest bare metal. I've used Johnsons paste wax in the yellow can for a lot of years to protect shop tools in a damp environment.
All of those cartridges are good. If you handload, there is no practical difference in the capability of the .38-55 vs. 375. Even when purchasing ammo, buffalo bore makes some really stout .38-55 loads. Since you are looking at a re-bore and re-chamber, I would ask the smith what is safe to shoot in it and what bore diameter he uses. The two cartridges are not identical and standard factory loads differ on bullet diameter and pressure.
I've always wanted a .307, but that doesn't solve your OH issues. I think that would also require more modifications to the rifle than just a re-chamber. It's close in dimension to the .308 case. I'm betting there will be some carrier and bolt face modifications with that. Whereas the .38-55 has a close kinship with the origins of the .30-30 and the conversion is comparably simple.
Sounds like a fun project.
