As far as I can remember there where two different 727 front drum sizes, stock and the 1966 and later street Hemi.
Do you have a factory service manual for your car year? if not find, buy or borrow one and use it for the correct specs, you may see "self proclaimed expert people" advising to add additional clutches to a "race" 727 transmission, that is not always the best way to go
As far as how much clearances you want between the clutch and ring retainer I would have to dig out one of the old Mopar race bibles to see what they recommended for each application, BB, SB and Hemi back in the day, which may not apply with todays parts
Are you going to use the stock valve body stock? If so make sure and get the pressures set correctly on start up
If your going to use a kit in the valve body or replace it use the manufactures recommendations on the complete set up