You are right , the calipers will not work , you have to buy a bracket kit to use larger calipers , but it still can be done , your a arms are for 71 to 73 only , they were made made larger because of the weight increase and heaver engines to come , the 74 up mustang II and pinto are the same front suspension parts , but if you brought a road race car you might want to check and see if the suspension has been modifided already , post some photos , someone can tell is it has ,