One of my mates is carrying out a 2.0 Zetec in a Dutch mk2 3door at the moment, you basically need to go down the same route as putting one in a mk1/2 escort or any thing else retro.
If you don't mind spending a little bit of money use the parts that 'retro ford' supply they do everything to convert to rwd. But with the sierra because it's basically the same as a 1800cvh you can use the standard crossmemeber and use the cvh engine mounts. This will keep all the engine and gearbox in the same place making it quiet straight forward.

Me and my old man also carried out this out on his cortina 1600e except we done it all for bout £300 !! With custom parts and different bits and pieces
Hope this helps mate