Hi Uggy,
First of all, welcome!
I'm not familiar with the Oz/Nz Ford/Mazda line up, is the 323 Front or Rear wheel drive?
If it's RWD it should be a fairly simple conversion, if it's FWD it'll be a matter of finding a RWD gearbox which will fit or making an adaptor plate to mate it to a suitable box. If you overcome this there's the rear axle to sort. The Sierra ues an independant set up, so you'll need to find a similar set up locally, possibly BMW?
Either that or switch to a live axle (possibly from a ute?) and use a Locost style set up.
Hope this helps mate
Cheers
Chris
BTW I deleted your other posts, you semed to have developed a stutter!