am i missing something here?

ive been researching the box4rocks doubler kit, wanting to do the 231/d300 combo. From what ive read, it should be aprox. 1/4" different than the length of a 231 with a sye, but when i went and measured my case the overall legth from mounting surface to the center of the rear ujoint cap is 16 1/2", the seperating part of the case is 7" back from the mounting surface, plus 1" in clocking ring and doubler plate, plus the 11 5/8" of the dana 300 (according to Novak) that puts the doubler legth at 19 5/8" thats 3 1/8" longer than the 231?? did i miss something, is my math off, or am i correct here?