Ulf, I believe the jack should be mounted with the hook below the mast, not above it. The spring you're showing is the correct one. In order of assembly, the spring goes on the jack handle, the jack handle tip is inserted through the loop on the floor and the lug wrench end nests in the formed bracket at the rear. The ratchet on the mast needs to be near the bottom of the mast. Mount the mast on the bracket by the decklid hinge and the lower end nested into the bracket at the rear with the lug wrench end of the jack handle,. The free end of the spring now hooks into the base of the ratchet where that loose spring is wound (the spring serves to help hold the hook in place on the ratchet.) The hook of this little spring should probably face the front of the car. Flip the lever on the ratchet itself to allow you to operate the handle on the ratchet where the jack handle would go when you're lifting the car. Keep doing this until you have enough tension in that little spring to hold everything securely in position.
As for the seats, I'm virtually certain that the seats were installed as a complete assembly. The seat back would have been laid over the cushion, picked up from below by the front and back of the cushion and dropped into the holes in the floor pan.
