So I swapped the finder of the 160 with the one of my donor camera, the 900. It was a breeze really. The mirror post for the rangefinder screws right in, I only needed to make shim to make it fit perfectly.
My Sinar back turned out to be useless.
The side facing the lens has a ridge that is very inconvenient. Don't buy one to put on your polaroid! I've decided to take the clamps of the sinar back and fix them to a DIY back.
I will make the back out of extruded aluminium profiles, and I will keep it as simple as possible. This allows me to tailor it to the polaroid.
Some progress pictures: