Mike:
Did you find an answer for this?
The .450/.400 x 3 1/4 BPE used lead bullets up to that diameter. The 'standard' bullet diameter was around .408, which was expected to obturate up to .411 groove diameter, but there is a lot of variability in bore/groove diameter in the .450/.400 Express.
There were also a couple of variants, such as the Fraser and Westley Richards cartridges, and at least one smaller rook-style .400 cartridge.
The problem is that lead bullet dimensions listed in cartridge texts rarely tell the full story with regard to actual groove diameter of the rifle they were chambered in!