im not sure what you mean about these case sizes,im not at all up on rear ends and their compatibility.i have a 1970 duster with no rear end.i want a posi 3.75 to 3.90 gears,tell me about the 3.91 and is sure grip a posi?is the 741 case the 3rd member only ?if so what else would i need to get?thank you.dan