The correct bullet diameter for the 450 BPE is .450. Wrap it with 11 to 16 lb paper and you'll be fine.
The original Kynoch and Eley bullets are this diameter, and the Kynoch factory drawings show a 1 in 12 alloy. Make sure you use only a lead/tin alloy, or the bullets will not obturate correctly.
They seem to work fine in any bore configuration I've tried, remember, a good double 450 is a 3-4" grouper at 100 yards. Plenty good for hunting.
While I prefer and recommend only Pb/Sn bullets generally, plenty of people have proven that antimony and non-zinc wheel weights can be used effectively (which might be helpful given the current state of Sn availability and price).
Personally, I prefer a 9# paper, but the 11# will be fine, IF the bullet and bore diameters are compatible with it. In the end, the best way to go is to pick the paper you like, measure it carefully (on a mandrel, for example) and then order a custom mould to fit. Then you might want to think about what sort of load you want. You can shoot anything from sub-land diameter to Groove+2 diameter depending on the shape of the throat and the way you want to manage fouling and handle your cartridges.