904??

I am interested in this 5 clutch drum swap, as I am in the middle of a 904 rebuild...Currently I have all the parts to rebuild what I have. (a 4 clutch from drum) How do I exactly tell what year my tranny is? It came out of my 68 cuda, but who knows if it's the original because my car had like 8 previous owners, and I am trying to unhack everything! haha! What other parts will I need, or what else will need to be changed? Is it totally necessary to upgrade to a 5 clutch front drum? How much will you gain?