I remember having similar problems when I tried installing 8.1 in Shapeshifter a long time ago. From what I recall, 8.0 worked with no problems but I never could get 8.1 to install properly. I think I might have been able to pseudo-install 8.1 by installing most of it's components manually overtop of my 8.0 install. Also, I think I had slightly more success with Fusion than Shapeshifter.
