With all due respect to Cal and Bud (actually I am surprised I caught you two) but ...
The original 427 Cobras used a Big in and a Small out (not the real small out that was recalled in the late 64 and early 65 cars) with a 1965 Mustang tailhousing. Please note that the Mustang tailhousing has to be bored out to put in the large output shaft.
I believe the big output shaft did not come out until 1968 with the 428 CJ Mustang.
For a 427 car you will need a Big Input. You do not need the big out put shaft unless you plan on drag racing the car. If you go drag racing please let us know which parts you break.
For a street car I would recomend a wide ratio trans.
For track use I would recomend a close ratio trans.
The wide ratio trans is not original but boy is it nice on the street.