I'm open to correction but i think if a certain amount of the car comes from the donor it can be done without an SVA(?) test and will just need an MOT and to comply with all the usual conditions. So if you have a 90(for example) chassis, engine, box etc and fit a different body on. Easiest route i would say is to base it on any ladder chassis'd 4x4 so SJ, 90, Disco etc etc
Its no help in how to do it but you might like this link :
http://forum.difflock.com/viewtopic.php?t=38942