Okay. I figured it out. So simple in retrospect.

I simply removed the two jumpers on the SIM7600G shield which link TX and RX to D7 and D8 since these are used to set up the SoftwareSerial.

I then connectd TX on the shield to Serial1 RX on the arduino and RX on the shield to Serial 1 TX on the Mega.  Changed the program to replace SoftwareSerial with Serial1  and loaded the sketch.  Worked first time. Onward!