There are also 2 reinforcements per seat on the inside of the floor one is also the tab for the arm rest. These are welded to the floor I cut them out of the parts car to install my buckets in my wagon.
I thought the 63/64 880 Police vehicles with 413, s came with this style of unsilenced air cleaner style. I have this on a 318 with an early small opening 4bbl carb.
I used the seats from a 64 Chrysler, there are some inner reinforcements in the inner floor for the inner seat studs. I cut this out the parts car I got the seats from.