Beardy

The best conversion i have seen of a focus was done by a guy called Andy Barker...(it is a rallycar) he grafted a suburu impreza sti into a focus, I am ford through and through but this was a pukka conversion and realitively straight forward. As Azreal says the best solution is to make a rear cradle for the diff to sit in, you will need to create turrets in the rear of the focus as you will be doing away with the beam and trailing arm set-up (as per esc cos/saph) and install custom suspension, i've seen a couple where standard front uprights have been used, 3 link bars from the bottom of the upright to the cradle and custom made drive shafts.

I think you could put a yb in longitunally but i'd be careful which box you fitted the mk5 escort rs2000 mates a 2lt with mt75 but you'd probably need to uprate gear kit, you can get a straight cut version for the inline mt75 but I ain't sure if it fits into the rs2000 box.

Hope this helps