Interesting question. The actual published base diameter of both is .544" while the .450/.400 3" rim is slightly smaller diameter at .614" versus .625" for the .333 Flanged. (inconsequential difference)

The only big difference is the rim thickness where the .333 Jeffery Flanged is published at .080" thick while the .450/.400 3" is .065".

The rim thickness would not be a great obstacle if the .333 conversion were headspaced on the shoulder or a .015"thick washer/spacer were fabricated. "Headspace" on the shoulder in any event will mitigate early case head separations.

Seems a doable conversion if one wanted to wait for a likely very expensive and long delivery time set of case forming dies.

Bertram makes .333 Jeffery Flanged brass. Probably less expense and trouble in the long run.
Not sure if anyone in the USA currently imports modern Kynoch ammo, but they do list the .333 Jeffery Flanged.

- Mike

EDIT: Just measured some several year old Hornady .450/.400 3" Jeffery factory ammo.
Base is ~.5425
Rim Diameter .620"
Rim Thickness .060"