in this picture
there is only thing that needs to pass through the firewall, and its not what you are holding, its the big connector to the left and below that has the threaded grommet.
you can trace that shape and make the firewall hole the same, then the threaded studs will hold it tight to the firewall and seal it.
I have done 3 trucks with 98-03 harnesses, everything else you are showing goes in or under the dash. there is a big fusebox/distribution that goes under the hood, and all the other underhood stuff/rear harnesses plug into it. I lost the pictures in my threads but I will dig some up to show you where and how I mounted it all.