I agree with the comments posted so far about proceeding carefully with your options.
I have 2 vintage doubles in 450 BPE. My Alexander Henry chambers modern brass just fine, but the other, an R T Pritchett, will not. I have not been successful with resizing Hornady and Kynoch brass in a .510 Lee bullet sizing die, as desribed in another forum here: http://forums.nitroexpress.com/show...;Words=+dracb&topic=&Search=true
Resizing the brass by removing the shell holder and pushing the brass further into the die did not work either.
I'm pondering what to do next- very hesitant about reaming chambers on a vintage rifle, so may go with custom dies - if I can manage to get them during this pandemic!
PS: Should you be so inclined, you can add your rifle to the Alexander Henry inventory post on this site.