Alternative solution.; start with what you HAVE to do no matter what...
. use a "something to protect cab from rust" product.
. use a "something to minimize heat transfer from engine" product.
Use your remaining money and time to instead of dynomat, sound deadners etc etc..... and get a personal communication system.
. buy a headset communication system
.... can get expensive but less labor than installing all the sound dampning stuff
. use cell phones- make pilot/co-pilot and kids communication system out of cell phones
.... Mesh Network (phone to phone) is what you use for this via an apps like firechat, zombie chat etc. along with some inexpensive or costly headphones or ear buds etc.
...... additional advantages
....... opens communication potential to folk in the bed or habitat on back of truck??
....... each user can play favorite music to their own ears
....... opens potential to communicate with others in your procession
....... protects hearing better if one uses sound dampening head set
....... relatively low cost / see below
..... disadvantage- you got to wear headset to use it.
https://www.geckoandfly.com/22562/chat-without-internet-connection-mesh-network/
https://www.amazon.com/Ryobi-Suppre...sr=1-8&keywords=hearing+protection+microphone