I don't think the body is different. Allthough it could be, in which case there should be a hidden program in the Cylon computers that run the downloading process, to make sure that those slightly different spare bodies (a sublty different brain or something) are used only for this special Cylon, and that this special Cylon never downloads into an "ordinary" 8 spare. Since the Cylons are programmed not to think about the 5, they might also have been programmed not to notice something like this (if they would even be inclined to pay close attention to how the whole download process is managed).
More likely, if she is the 5th, the difference lies in the part that is uploaded/downloaded, or there is no difference at all (that is, Cylon model 8 is a carbon copy of one of the Final Five - a bit like a human who gets cloned), other then Boomer's origin and age (and experiences). She would be an early prototype, in that case.