As Ron has described/suggested, the Jim Carter upholstery installation kit is woefully inadequate and does not match the number of layers and the thickness/firmness of the original installation.
One missing layer is the stiff coir matting, which can be somewhat matched with piece of strong very short nap carpet.
Denny Graham did a nice photo write-up on a good installation. He might have found coir matting or a synthetic equivalent. I'll see if I can find his posts here.
Even with the correct layers, the old seats will never be as comfortable as the seat cushioning in a modern vehicle.
