Graeme Wright had the same issue with one of his rifles in .375 x 2 1/2" with an oversized bore so he got die made to swage up factory projectiles.
So I guess D you can try that as well or resize down .277" projectiles.
If you swaged down you'd have probably 3 steps to do. If you could get your gunsmith to run you off a few you may be able to see if there is life left in the barrel using correct sized projectiles, certainly doable.
The only other option besides rebarrelling will be casting your own.
just my 2c's worth