P100 had a live axle, Sierra's have a diff and trailing arms.
You would have to fabricate new mountings to fit the beam, I would look at fabricating a cage to hold the diff and wishbones, have a look at the Cris Gibbs book "How to build a sports car on a budget" it has all the dimensions for the diff, wishbones and hub carriers.
It looks a lot of work but will be less bulky than trying to retro fit the diff carrier and weighs a load less.
Other option would be to upgrade the internals of a P100 axle add trailing links and panhard rod (or Watts linkage) like the escort rally boys do. Most bits available from Rally Design.