First of all, thanks for your answers. I ended up dropping the idea of modding that system and told the seller I would not get that iMac after all.
Now hopefully I don't anger anyone asking this, but I want to know if I have the right idea to commit the money. I've gotten hold of a fully working 2011 27" iMac with the 6970M. Since I want Metal support I've been reading as much as I can and decided that my best course of action, considering my budget($50), skills with Dremel tools (none) and willingness to mess with software is to settle for a Red Dell FirePro M5100 (note: would have liked a WX4130, but it's outside my budget). I know that it still has some issues, but I would disable sleep/wake and deal with the NVRAM corruption if that's my luck.
Having said that, I'm a bit confused. Considering I'm on OCLP v. 0.4.4 running Monterey, My plan of attack consists of before doing anything add the CAIL lines in this post:
Experimental M5100 Support to my config.plist and set the type of GPU on OCLP.
Open up the iMac and switch the graphics card. Put it mostly back together.
Power up the iMac to see if the graphics card work.
This is where my confusion pops up. If I understand right, in theory I should be able to access Monterey (the OS I have installed atm) right away?
If that's the case, after that I would use the Windows 7 bootcamp partition linked on post #1 and install the vBIOS for the card.
Reboot into macOS, finish putting it back together.
???
Profit
Barring the card arriving DOA, that would be the gist of it?
Side question: watching videos I've learned to be very fearful of the power supply. What precautions should I have in order to minimize risk? Power off the iMac as much as I can using the power button and using rubber gloves while removing the screen?