I would add entire rear alxe as well - might as well try to use the front brakes and the electronics
Problem is, you won't want to use the Omega rear subframe and trailing arms, which must weigh a ton, so you're into designing a chassis around the diff and hub carriers. You may as well just use the Ford bits that someone else has already engineered a chassis around. Sierra diffs are bomb proof and plentiful, including LSDs, which is obviously a requirement. 8-)
The engine and box is a different matter. You just have 2 engine mounts and a gearbox crossmember to bolt on somewhere.
I've seen a Westfield running an X30XE before. Looked a nice fit.
I would also look carefully at the weights of all the available engines and boxes first. I suspect you could do better than the Omega bits and weight is all important when the engine and gearbox is going to be a third of the weight of the car or more.
kevin