I know this is almost an apples to oranges comparison, but I think it will help.
My 1952 M35 Gasser with the stock REO OA331 and a muffler with the tail pipe out the back between the duals is hardly heard when sitting still. Above 30 mph or so, it is just as loud as my M35A2 multifuel with the whistler turbo. Hearing protection needed for both trucks.
How it applies to you is 3 axles turning, 1 transfer case and a transmission also turning. All making noise and heat. Then you have the square truck with air brakes for mirrors out in the wind trying to move through the air. A flat windshield will geneate a lot of wind noise. I think engine exhaust is just a small part of the noise inside the cab.
That is my duece noise example. 6.2 noise example is this. 1984 former USAF Suburban with a 6.2 and all the factory insulation. Hood, floor, doors, firewall and a headliner. I can't hear the engine until I am over 45-50 mph. Then the diesel sound from the engine itself is heard. Not annoying, just there.
1985 M1009 with no insulation and the 6.2 is a constant diesel sound. However, I can still hear the wind in the mirrors above 45 mph so it isn't that bad of a noise.
If you set the IP so the lines are lined up, the engine will make a lot less diesel type noises. Advance the IP toward the drivers side and the noise level goes way up.
I really doubt your truck will be signifigantly louder at speed with the 6.2 than it would have been with the 302 gasser. Driving in a parking lot is another story.
