MK5? MK6? I'm a bit confused which car(s) you're really talking about AND what you want to do?

The 2002 Fiesta already came with rear disk brakes (ST 2.0, at least Germany, Austria and UK, some countries still had drums fitted), so there's no use trying to make an axle of a later car fit as it NEVER will be as good as the standard one.

And on earlier models theres also an easier way to change from drums to disks than to make a different axle fit.