450#2
I thought about the 444 as well but there is really only a marginal amount of capacity increase over the 375 Win.
Pressure wise, the more case capacity the better and that was why I thought the 405 or 45/70 may be a better option.
If you are going to all the trouble of rechambering a double and making/modifying dies/cases etc the end result would have to be a substancial improvement over the original chambering to make it worth the time, effort and cost IMO.