The I2C bus can do 0.5 meters.
0.5 meters for 64 Slaves, that is 8 millimeters per Slave :o
Thanks for explaining the project. That is a lot of pins. The I2C is a slow bus and in my opinion not suitable for this task. Using cascaded shift registers is more straightforward, with some input protection.
64 Slaves in software might be a problem as well.
Try to make the project flexible and allow to replace the Arduino Mega 2560 with an Arduino Due.
The I2C bus was invented by Philips to store the settings in a EEPROM inside a television on the same pcb board.