1) You cannot use an AD Piston for anything other than an AD injector. You cannot use it with any single shot P&B including the 530's. You can only do very minor machining to the AD Piston to get a small increase in fuel from them, usually not worth the trouble, that's why a lot of guys convert to Singles. The split shot timing is very touchy and you generally do not want to mess with the Piston length.
2) You can use AB or AC Pistons without machining with a Single Shot P&B, including the 530's.
3) When you convert a AD into an AA you must replace the AD PIston, too. See 1) above...