Sim800l Atmega328p voltage level translation

As I understood from the datasheets i need to use a level shifter between sim800l an Atmega but i dont know how to find the values for the resistors R5, R6, R8

I dont think you will need those resistors. As level translation is taken care by the chip.

What if i use the same power source for both of them the level translation is not needed i guess?