Ford did devolp a 4WD kit for the RS Turbo,but it was shelved,due to unreliability issues and also other projects that were deemed more important IIRC...
Way back in the 80's i knew a guy that was going to buy this very kit for his big powered Series 1...a kit was found in Germany and it wasn't cheap,but it was incomplete and to get the missing parts remade would have cost alot,so the idea was forgotten about.

I'm sure if you dig around in one of the respected Ford writers book (Graham Robson etc) you will find some info on this project.