15+ Arduino Mega Pinout Pictures. There are pin mappings to atmega8 and atmega 168/328 as well. First of all, why arduino mega 2560?

Mega2560 Pro Mini Epalsite Wiki
Mega2560 Pro Mini Epalsite Wiki from wiki.epalsite.com
As described earlier that the arduino mega is based on the atmega 2560 the pinout of the atmega 2560. There are pin mappings to atmega8 and atmega 168/328 as well. It has 54 digital input/output pins (of which 14 can be used as pwm outputs).

I realized that the pinout wasn't the same with grbl for both boards.

You can get the yourduino mega2560 here. When cheaper boards are available, why go with arduino mega? There are pin mappings to atmega8 and atmega 168/328 as well. Each pin comes with a specific function associated with it.