Thanks for the quick responses.
The specified ways work excellently - but - I neglected to mention that the boxes have been generated programatically from a little level editor Im working on. I generate the level - load in Ac3D and optimise.
I dont fancy doing the whole level by hand :S
I was being lazy and not optimising the boxes as they were being put onto the level - I'll spend some time optimising (programatically) before loading it into Ac3D.