One of the big mistakes that most builders of pickups do is to put the mufflers under the cab and exhaust pipes exiting near the rear wheels.
I generally have the mufflers on a pickup, regardless of what kind of mufflers they are, placed as far under the bed, away from the passenger area, with the pipes extending out under the rear bumper.
The mufflers on my '64 are chambered stock type with 2-1/4" pipes running all the way to the bumper. The inside of my '64 is very quiet, I used a lot of Danamat and/or Road-Kil on the floors etc. I also added weatherstrip rubber on the upper edge of the door where it meets the A pillar.. The Delux C series AC equipped camper specials came with this added door seal.
Of course when the truck gets above 65 mph the wind noise really picks up.
I generally drive my '64 with the windows up and the AC on, makes a big difference in the wind noise.
