My Cogswell & Harrison carries a "52" bore Birmingham proof mark which is .447" bore. (Means the barrel at proof would take a "52" gauge rather than the true .450" 51 gauge) The 7 groove standard Baker style rifling measures ~.459" to the bottom of the grooves at the breech end.
As I mentioned, the chamber/cartridge specs you see in the drawings (and as Ben mentions) are very likely based on "worst case" of having to accomodate an early rolled brass cartridge. Those necks were considerably thicker than drawn brass cases. Plus, it is very likely that, as with the generously cut military chambers, the chambers are cut to allow dirty/corroded/bent cases to easily chamber and extract. Except for target shooting, British ammo just was not reloaded when out hunting so excessive expansion and case life were not an issue.

Rolled brass case:
http://40.media.tumblr.com/336b7856d0cd679a0fabdc184523e912/tumblr_inline_nlqpazVhM41qapn73_500.jpg

So....no you do not need a weird non-standard size barrel for any of the .450's, just accept a sloppy chamber if using the antique drawings as a guide.

- Mike