'73 Disc conversion, but keep 4" bolt pattern

You're correct, the drum is swaged to the hub with the wheel studs. To install the Scarebird kit, you cut or grind off the wheel studs, knock the drum off, then press the old stud remnants out of the hub. Press in new studs, and you're good to go. You also have to drill the rotors for SBP, and sometimes you have to grind the caliper a bit to clear your rims. It ain't hard, but it's an afternoon of dirty work.