I believe that only 1Ds/1D bodies will focus (AF) with the 100-400 + 1.4x extender. It would give you a f/8 and 5D can only autofocus up to f/5.6.
Edit: it might be possible to autofocus if you tape the contact pins on the lens but AF will be really slow.
Taping the pins will not solve the fundamental problem that at least part of the AF senors will not be able to "see"
out of the lens because the spacing of the sensors demands at least a f/5.6 opening. The AF of the 1D cameras
have better sensors that are closer spaced (and compensate by being more accurate) to be able to give AF with
only a f/8 opening.
See the
Phase detection section to understand why taping the pins will not work reliably.
Ronny