Okay well I was browsing CI (again) and I came across "1975-79 SEAT MOUNT ADAPTER LH" for a Chevrolet Nova.

http://www.classicindustries.com/nova/parts/81186.html

Now I look at this picture and I wonder how does this work? I see mounting points on the bottom but I dont get how this works with a seat.

Can someone clarify please? Thank yuu!

That looks like a bracket for adding after market seat. You need them for some racing seats.

This adaptor allows a seat to be bolted to the floor pan. This appears to be for use with a single bucket seat and not a bench seat. This might be for either aftermarket or stock seats. I would try to get clarity from Classic Industries first, if you are thinking of buying it.