There are a few issues that make this very hard!
As you can see here Apple dropped NVIDIA MXM board support around 2010 [https://everymac.com/systems/apple/imac/imac-aluminum-faq/imac-intel-aluminum-video-processor-upgrade-capabilities.html|iMac Video MXM bd options] so your system will run in compatibility mode using the older 2009 driver. You might be able to get a newer driver from NVIDIA directly as there was rumbling of them creating one quite a few years ago.
The next issue is the boards firmware is unique to Apple MXM boards so you would need to figure out how to find the older NVIDIA board firmware and if you can alter it to work with the GTX 1070 board.
The next issue it the heatsink getting the correct one so you don’t cook it or your system.
Is this worth the effort?