Unless the carpet in your replica is already the correct color (doeskin) and is in good condition, you will probably need to replace both the carpet and the underlay.The underlay acts as a barrier between the frame and the carpet, insulating it from outside temperatures, moisture, rust, oil, and noise.
It is a good idea to replace the underlay when you replace the carpet, as it will help keep your carpet stay looking new. Newer underlays also provide additional noise reduction.
1982 Pontiac Firebirds (like KITT and KARR) had cut pile carpet, and it is best to have it pre-formed to make installation easier. Carpet is very hard to work with when it is cold, so make sure to warm it up a bit before trying to work with it. Leave it in the sunshine or use heat lamps to make it pliable.
Many carpet kits come in two sections: the passenger area, and the cargo hatch area. Make sure you order both at the same time from the same company. This will ensure that their hue and brightness will match. If possible, ask that the supplier match both before they ship them to you.