In an inverted vee the prop shaft is shifted down relative to the crankshaft, which is at the top. In an upright vee the prop shaft is shifted up from the crankshaft, which is mounted at the bottom.
The annular radiator wouldn't know if it were an upright or an inverted V-12 behind it.
As regards to the Spitfire and Mustang, if they were to have an annular radiator the engine and thrust line would have to be moved down.
Moving the thrust line of a upright-vee engine enough to accommodate an annular radiator, in the way you're suggesting, means the entire airframe would have to be redesigned, all for very little gain in efficiency or performance. Plus, for the likes of the Spitfire, without an almost complete redesign of the airframe, the cg would move significantly. In practice, during wartime no manufacturer would risk such a radical redesign.