Ian,
Actually I already have the FRP seat backs, and the squabs for these seats rest on a webbing trampoline made of Pirelli webbing which is sprung directly to the chassis of the car. There is no bolt-in seat usable in a Mark I GT40 chassis, at least not that I have seen. It's an unusual arrangement- the seat back is padded and upholstered, and the seating material is continuous with the squab part of the seat, which just snaps down onto the trampoline. If you look at the drawings of parts in the old GT40 parts catalog, what you are looking at is just a fabric and padding construction that gets its shape from what it rests on, for the lower portion of the seat.
As far as the FRP upper part goes, in road cars they were held to the back bulkhead of the passenger compartment by a bolt, but to make them easier to remove, Ron suggested we use steel tabs and Dzus fasteners, which have worked out very well. (photos later, I saw them today). For the FRP parts, there is a lacing across that part through eyelets which have to be installed- then the upholstered part is installed over that. (this is difficult to describe, photos do much better, I will see what I can find. There are some pretty good photos of some of this on Jay Cushman's web site)
Finally, virtually all GT40 interiors had the ballistic-skirt cloth upholstery with the grommets. I've opted for an upgraded interior which will look like the one in 1043; as I understand it, this was an option in this car from new. The seats do not have the grommets, and they are partly done in leather and partly vinyl. (I had leather made up to match those two materials). We'll see how it all turns out.
