SPI: 50 (MISO), 51 (MOSI), 52 (SCK), 53 (SS). These pins support SPI communication using the SPI library. The SPI pins are also broken out on the ICSP header, which is physically compatible with the Uno, Duemilanove and Diecimila.
I understand that the pins are not arbitrary,
Sorry, I really need to explain myself better it seems. The idea would be to connect the SPI pins from the ATmega2560 to the same header pins where SPI lines are on the UNO. PB3 (MISO) to header Pin 12PB2 (MOSI) to header Pin 11PB1 (SCK) to header Pin 13PB0 (SS) to header Pin 10