I'm in the same boat with my '86. I installed a turbo and 4 inch exhaust with a resonator, and while the engine sounds great, not much of a fan of it on long drives. The issue I found with these trucks is the complete lack of insulation or sound deadening on the sheet metal, which results in driving around in a tin drum! I pulled and refinished my interior this summer, and before reinstalling, covered the bare roof panels, rear quarters, rear panel under the glass ect with self adhesive Frost King duct insulation. The product is a foil backed foam, made to insulate and quiet hvac ducts. Three rolls took care of the cab, and cost about 60 bucks. It cut the harmonic vibrations, was easy to install, and also helps insulate the cab from heat, and hopefully cold. I am going to step up and install dyna- mat on the floor around the exhaust when I pull the carpet, and get a new hood blanket when I install my new(er) front clip. Also take the time to tighten every screw and bolt in the cab that you possibly can. Pay attention to your roof support ribs, as the caulking (dum-dum) degrades and separates, causing the roof to reverberate. I cleaned and siliconed mine. My old girl has zillions of miles and looks like she escaped the wrecking yard, but currently has no rattles from the interior. Just takes some time and patience.