1) did you do a chamber cast or other technique to confirm that your rifle really is a .450 3 1/4”?
2) “smoke” the case (“lampblack” or whatever) and rechamber to see where the fit is too close.
3) get out your vernier calipers and compare case head dimensions between your new brass and the originals.
-or-
4) just sell the rifle to me, I will dispose of it properly I promise.

NOTE:
There were no “standardized” case/ammo dimensions in the 1800’s, that only happened in the early 1900’s. Could be that you have a tight chamber. Custom sizing die may be one option.

- Mike