Well that sucks, but, something to think about and research if you choose, I'd be on the phone with CEB bullets first thing Monday morning, the 650gr CEB Safari flat nosed solid would do all you ever need done, Michael McCoury has done decades of exhaustive as well as expensive pressure testing research, the newer banded [read bore rider] mono solids from either CEB or North Fork are indeed kinder to old barrel steel.
I shoot the 450gr North Forks from a 1906 year model Holland double without concern, as well as the 210gr CEB solid in a 1900 year model Rodda 303 British double with fluid steel barrels, no damage, no harm, as well as very little fouling.
Pick your shots as not to kill or injure a second animal and You're Gold, have seen it first hand off the end of my own 577 double, "NOTHING" lives long with an inch and a half garden hose sized hole blasted clean through, nothing.
Again, good luck to you Sir, hope you get to carry that fine old rifle and put her to work.