It's been a number of years since I put it in the truck I drove at the time ('96 Ram 2500), but I purchased Dynamat and it worked really well. I have heard that you can use discount stuff just as well, but be careful not to go the super discount routes as apparently the adhesive/ rubber may start to smell like asphalt. I believe Eastwood makes some now that may be cheaper than Dynamat.
If you layer the whole floor with it, it will be the best, but obviously more expensive. The whole point is to stop the vibration of the panels and therefore dissipate the energy of outside sound/ vibrations before it gets into the cabin, so you can cover only some of the panel (enough to stop most vibrations) and it will be nearly as effective.
Once that is done you can put a thin layer of jute or other sound absorbing material over this to further absorb anything that gets through and then put the carpet back.
I would suggest doing it to your doors and under other panels too. You'll be surprised how much it helps. It's a great investment- muscle cars and other classics are louder than you think. You only get one set of ears, and trust me, when you have hearing problems you would do anything to have prevented it.
