GSM SoftSerial

Hi,
i would like to change the RX Pin of the Gsm Shield to Pin 53of the Arduino Mega.... So far a edited the GSM3SoftSerial.cpp

#elif defined(__AVR_ATmega2560__) || defined(__AVR_ATmega1280__)
#define __TXPIN__ 3
#define __RXPIN__ 53
#define __RXINT__ 4

and i conncted the GSM Tx Pin to the 53 oin of the mega but i still stuck in with connecting to gsm... ?

do i have to adjust the RXINt in anyway ?
Thanks in advance