Danger, danger!
If you have pads that are 20 mm (2 cm!!) too "long", you have the wrong pads. This invites the question as to which were wrong.. the previous or the new ones. There are at least three different pad sizes: '79-84, LS, and '85 on. I suggest comparing the pad wear area against the rotor to confirm. (I'm talking about front brakes here.. not sure if OP meant front or rear.)
Bedding in, several quick stops from some speed, starting at 25 mph up to 40 mph, with "breaks" so that the rotor and pads do not overheat.