There's a few people building MX5 based roadsters (ie using uprights & diff's) so I'd be tempted to use the whole lot & forget the Sierra.
Rumour is there's going to be an updated version of the book with this kind of info in....?
If you're not interested I'll take the whole car off your hands
EDIT: Here you go:
http://www.mx5-locost.co.uk/
Its Airhead on here (I think!)