Why doesn't my billet front drum fit?

If it's the same width as the original drum I'd inspect the back side where it mates up to the rear drum. 727's used 3 or 4 different designs of the rear drum and they don't always mate up to the front drum correctly. If I'm not mistaken most of the aftermarket parts are designed in mind for the early 70's 727's. If the rear drum is a late 70's unit, or even a 60's unit there may be a compatibility issue.