In this video, I remove the rear wall panel, insulate with sound proofing and continue with the riv-nut conversion, finishing off both cab doors, adding soundproofing as well.

I want to share with you what a certain subscriber shared with me. There is some experience and wisdom in there and if you want to figure out the true anatomy of sound proofing, it is a good read.

"elgoog Sucks" writes: "Tl;dnr - if you dont mind these materials inside your truck, asphalt roll on 30-40% of the metal, some dense padding if you have it and pink fiberglass in the empty spaces.
I have watched over a dozen youtube videos, so I am what you could call an expert..
Kidding aside, this is one of these rare instances where I do actually have some experience in the subject I am about to comment on. : ) Firstly, a 3db change cuts the sound energy in half, but a 10db change sounds like cutting the volume in half to our ears. So a 10db reduction will sound to your ears as if you cut the cab noise in half. To cut it 20db would be to cut it in half again, so it would be a quarter as loud as when you started. It has to do with how our ears work and it is why 40db is a quiet library, 90db is like standing in traffic and 120db causes hearing loss over time.
There are people who are PhDs in this stuff, and I am not one of them (despite the vast YouTube experience I mentioned earlier). As a practical approach though, you want to do something to stop the drum like vibrating of the steel and something to stop the sound as it moves through the air afterward. I saw a guy use a roll of asphalt to stop the vibrating on his BMW and it worked well. He even did a follow up video a year later and showed it did not fall off and did not smell. That stuff is $20 a roll at Home Depot. I also read that it has diminishing returns (putting you finger on a drum head and hitting the drum stops a lot of the noise, putting your hand on the drum covers a lot more area but doesn"t reduce the noise that much more).
Next you have to deal with the air inside the cab. High and low frequencies act a little different. Low frequencies are absorbed by dense material. Thick, dense foam pads work well. Higher frequencies are absorbed by that too, but are also readily absorbed by lighter stuff like fiberglass insulation. It helps to have as many changes in medium as you can get (a million little pockets that the sound has to convert from air to some material back to air on the other side).
Lastly, it will help to deaden the interior of the cab by absorbing any reflected sound. Carpeting like you are already doing should work great.
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https://www.youtube.com/watch?v=vN88_VQMVVw