Although the rumors (and of course they are just rumors at this point) say the testbed body is 1D-style, I'd guess any production 3D model will be a souped-up, fully weather-sealed version of the 5D Mkiii body. (Maybe even with eye-control again!)
If you're going to give credence to this rumor, you might also give credence to the thought that Canon might use an existing body to test new internals, such as a new sensor and new electronics. Also, the larger body gives more room inside to rig stuff up and test without having to wait on the body to be ready to go. And there is a "camouflage factor" also.
Of course, it could be they've changed their minds, and decided to bring out a 1Ds V.