Why are you upgrading with 2wd Cosworth calipers to start with? They don't fit the standard Sierra knuckle, as the thread on a standard Sierra is in the caliper, but on the 2wd Cosworth caliper it is on the knuckle. So when trying to fit a 2wd Cosworth caliper to a standard Sierra knuckle there won't be any thread in either hole. Besides that, the bolt holes are not the same, M10 on a standard Sierra, M12 on the Cosworth and I believe the spacing between the caliper's ears is also very slightly different.

If you want better brakes there are much better and lighter 4 pot calipers around than the 2wd Cosworth.