Sorry for spamming your discussion area, but I thought someone might find the solution helpful.
The frameworks referenced by the 360 build of the Box2D.XNA library were refering to the 18.104.22.168 versions of the frameworks, and not the 22.214.171.124 versions.
Not sure if this is a fault with Visual Studio trying to fill in blanks, or the conversion process from PC to 360?
Deleting the previous framework references and replacing them (Microsoft.Xna.Framwork, mscorlib, System & System.Core) with the correct versions resolved the compilation error.
The references in the 360 test bed were all correct.
An interesting learning experience!
Sorry again for talking to myself on your space!