I have made documentation of the 670MX upgrade in the G75VW
here. It'll give you an idea of what you need to do going forward with another GPU instead (it's not just for the 670MX upgrade).
You'd need the 880M AND the heatsink assembly for it as well. Then, relocate everything off the 880M plate and take everything off the plate of the 660M and line it all up onto the 880M plate. You also don't want to keep heat there too long (soldering or heatgun) because it CAN cause the heatpipes to expand and look blown up, reducing the effectiveness of heat transfer (wicking) as well as physically limiting space for the entire assembly to sit flush back into the chassis again. Just some things to take into consideration.
Integrating the 880M VBIOS isn't the problem, as the VBIOS was read on the G750's GPUs unlike the G75's which is through the system BIOS. So you'll still be required to insert the VBIOS to get it working correctly. Likely due to the size, you'll have to remove the 660M AND 670M in the system BIOS as well so you have just the one (880M). Then make a backup BIOS image with just the 660M inserted so in the event you need to do troubleshooting, you can just install the 660M and perform a recovery. Otherwise you'll be playing in the dark, pressing buttons and cursing incessantly. Voltage won't be an issue either, you can just download the modified 880M VBIOS off my site and use that one for inserting into the system BIOS of your G75. The stock voltage on those cards (as is with any GPU) fall within a range between A and B, before it's flagged and binned as a lower end model (or higher end model) for not passing required field tests. The G750JZ (880M) models had the 230W adapters. The 180W adapter may limit the boost clock on the 880M (tricky little GPU to play around with) because "Pwr" and "vRel" is what limits boost with that card (as seen in GPU-Z), but take that road when you get there. One step at a time.
If you got any questions along the way, you can just PM me at my site or here.