I would choose a bullet of .327" diameter, and likely that will mean shooting this rifle as a breech seater because a .327" bullet wont usually chamber as fixed ammunition.
I have a Hepburn Match B with a .327" groove, and I shoot a tapered .328" bullet breech seated. I tried a .327" fixed cartridge, and it wont even come close to chambering. I had Jerry Cleave build me a Schoyen style breech seater for my Hepburn, and it's extremely accurate this way.