[Brian]

As you've figured, the seat base can't be a flat wooden arrangement, because that area of the floor ain't flat. Only the seat back is on a wooden frame. I do have a spare seat base, but it's going to be a mission to get it out to photograph. If no one else responds to your quest, get onto me and I'll do it. The seat base is a pressed steel affair that follows that floor plan detail and locates on the two 'pins' protruding up outta the floor. The pressed steel affair encases a woven fibre sortta material for the fabric to be tacked to. Lighter channels are crimped onto the springs
As far as I know, the touring sedans also ran the same seat BASE, you'll be looking for one I assume.