On standard Sierra (so also GT) knuckles there is no thread, the thread is in the calipers. On 2wd cosworth this is the other way around. But normally it shouldn't be a problem to fit 4x4 cosworth calipers to standard Sierra knuckles. The only thing is that you need 278 mm discs with the correct offset, which don't exist. The best option is to use 283 mm 2wd cosworth discs and file the edge of the caliper to give it some more clearance (it will touch if the disc heats up and expands). You will also need some washers between caliper and knuckle as the offset is still not exactly correct.