That's not a 2.6" vs 2.75" piston problem. Both of those calipers are more than capable of locking up the brakes and stopping the car with any of the stock bore size master cylinders, from 15/16" up to 1-1/32". Probably a wider range than that too, that's just what I know for sure.
I sure would like to find it I have not tried replacing pads yet just bought this car not sure if this was always a issue or just started old owner not much help
