By the sounds, money isnt much of an object to you, so I would suggest finding a late model Cortina Diff with ratios to suit your application, and get it shortened by an engineer. I did this to a mk4 diff and it set me back $500 from memory, makes a HUGE difference to a 2L 5Speed Conversion, I can tell you!
Also, when I had it cut, I got them to take an extra inch half out (cant remember ) of it so now I can wedge wider wheels under it and screw it down low without fear of scraping.
Depending on the length of your 6 speed (dont know anything about them), you can get away with a Ford Driveshaft combo deal. If its the same length as a 5speed box, the front portion of a 4speed Escort D/S and the front portion of a 5sp Sierra D/S will mate up perfectly.
Engine Code? I know the 2L Cossies are YB's, am I correct people?