CJF, are the chambers 3 1/4"?
270 grain bullets was the standard for the 450 BPE. I just bought an original box of Kynock blackpowder rounds. The bullets are 270 grain hollow points filled with copper tubes. And measure .454" diameter over the paper patches. My Thomas Hepplestone was regulated for 120 grains of black. Also I suggest you measure your bore diameter breech and muzzle as it could be choked.