Sedan seats may very well fit but there will be no tilting foward of the backrest. Also, sedan seats do not have the fore/aft travel length of coupe seats. You may as well forget the back seat if sedan seats are installed.
I don't think the 126 coupe seats would work. The seats are much bigger if I remember correctly. I had a friend with a 560 SEC and seem to remember they were much wider. You also may need to think about the controls for the seat and whether they are compatible.
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