Ahhh I'm an idiot. You can't have a 120hz display on the m18x R2 as there is no eDP connector like there is on the 17 R1.
Are you getting another M18x R2 or an 18 R1 (also known as the M18x R3)?
Yeah, you need to mod the heatsink in order to get the MSI 1070 to work. There is a guy on Ebay who sells pre-modded heatsinks for the M18x R2 and M18x R3 that have the area cut out for the MSI card. They are also good as they usually have 6 heatpipes instead of 3. He custom makes them and they are ALL copper which is good for heat dissipation, his name is Cicichen, or just look up Alienware M18x R2 6 heat pipes.
I couldn't really help you there as MXM slots are very complex. Your best bet is taking it into a shop sadly.