8 3/4 with a /6 and a 904 (drive-shaft needed)

I have an 8 3/4 with a 741 casting and the small yoke on it - If I'm not mistaken, I can use the current shaft that is used w the 7 1/4, I just need to shorten it? If this is correct, what is the correct size; 3 or 3.5 inches shorter?

Thanks in advance.