Alan, I had a 1977 F250 Super Cab (Ranger, XLT, Camper Special, if you want the full title)
I did some dB meter testing inside the cab to get a baseline and with the windows up, going down the highway at 60-65 mph, it was in the 98dB-100dB range.
This was a solid truck, in good condition, with good door seals and such.
Then I started messing around with things to get it quieted down. I started with Dynamat Xtreme.
I covered the entire roof where the interior trim and roof panel was located.
I covered the entire floor, including the extended cab area.
I covered the insides of the doors... Lots of Dynamat used in many areas.
This did produce some sound deadening, but the measurements were still in the 92dB range... meaning still loud while going down the highway.
I had highway rib tires by General (10 ply 265/75-16), and they were not loud in the least.
I did have a performance exhaust, but I built that to be quiet, so it was not too loud. Hooker Super Comp long tube headers, dual 3" pipes going into a single 5" mixing chamber, then into dual 3" pipes again, into dual GIANT Dynomax Super Turbo mufflers (3"in, 3" out) and then into dual 3" tailpipes exiting at the passenger rear quarter between the rear tire and the bumper. It flowed great but was very quiet too.
Still, the truck was loud, so I began trying to figure out why.
Through many iterations of covering, removing, and testing I found the major noise was plain old wind noise across all the various high spots of the trucks design.
Rain gutter noise, windshield trim noise, cowl vent noise... just pick a spot because these trucks are like a porcupine with all the things sticking into the air just enough to make wind noise.
But the worst, by far, were the extendable towing mirrors mounted on the doors with the factory bridge girder system... You know the style with the four mounts on the doors using stainless steel tubing.
I removed those as a test one day and BAM! Holy shit! The truck was suddenly quiet. It was not sedan quiet, but it was no longer so loud you needed to speak way up to talk with passengers.
I never would have guessed the mirror mounting system alone was making THAT much noise.
My advice, install the sound deadening materials because they do remove a good amount of general noise in these old trucks.
If you can, install a transmission/rear gears/tires that allow you to run down the highway around 2100-2300 rpm at speeds you will cruise most of the time (around here that is 65 mph)
This lowers the clatter and general noise of the truck too.
But, if your truck has the big eared towing mirrors, and you are looking to quiet things down, get those off the truck to decrease the interior noise immediately.
My problem, at least on an F250, is I like the look of the towing mirrors, especially the tall thin "California" style towing mirrors that would have been on the 1967.
