Since you have a 7424 (I assume, with the factory 5" counterweight) I would buy a lighter backing plate like a 5" diameter one or preferably the
Meguiar's 5" plate. A 6" backing plate with a 5" weight would probably vibrate horribly, especially once you actually put a pad onto it.
Check my thread below for some info and the thread type of the pad you'll need.