Thats the basic idea - but use one of these with a Hotbodies diff off of ebay...
Turn it 90 deg so that you can mount the motor as you have it in the pics. Then you just need an extension to make the mount hit the sides.
Get a "normal" diff mount for the other side in the same manner.
I would then add some additional bracing between the diff mounts...
---------------------
...........|_|
=====
|O|
...........| |
---------------------
Ignore the ...
=== is the motor
O is the diff
bold is the diff holder