I would have thought that if you have a blow out on your front wheel that it would be best to put the good wheel from the back on the front and then the spare at the back. I would have thought that any differences in wheel sides on the front interfere with the traction control / electronics. Realise that you wouldn't be exceeding 50 or should I say 40mph and that the spare is only so that you can 'limp' home.
and how is having a bigger, yet still not wide enough spare going to change things