Anything you can do to seal out wind, road, and engine noise will be sound proofing, and there are a number of ways to do it.
One that we all found that worked well was a product called 'Peal N Seal'. It is an asphalt based product backed on to a foil. You place it on the cab floor (or anywhere else) and it works like 'dynamat' but it is much cheaper and it sticks where you put it. I used it under my heavy duty rubber mat on my 59 and it was a big improvement.
This time I got a big roll of insulation to go with it that is also aluminum foil sealed on both sides for my 72 truck.
Carpet helps - really deadends the sound, and the trucks also had a jute mat that got stuck to the firewall on the inside of the truck above your feet.
You can also spray on a heavy undercoating/sound deadner on the bottom of the truck and that will help as well.
On the various luxury cars I have had (such as buicks and lincolns) you often found 1-2" insulation stuck under the hood to dampen engine noise.
And all of that stuff can be put behind the in cab gas tank to further deaden noise as the truck is a big sheet metal shell and does resonate pretty well.
Then you can get quieter mufflers, make sure the door windlace (seals) and window channel felt seals are in good shape and that the rubber grommets on the fire wall are all in good shape and are sealing up.
That's what I can think of off hand. I have noticed that newer vehicles have no exposed cab metal now, and so its obvious that all that covering material must also make the thing quieter.
