Hi,
I have had similar style seats in Moke before and each case I made up adaptor plates, no sliders though, to position the seats where I wanted them as that was the easiest way. If the seat belts aren't attached to the seats then in most cases a sturdy base is okay for the seats but there are DR info sheets that talk about the official requirements for seat mounts if you want to do some googling for the Federal or State level information.
Terry