Charlie,
With a 6-cylinder Chevrolet 261, I am using Patrick's stainless steel dual-exhaust system muffler (one muffler with two-inlets and two outlets) and it is too loud in the cab for me.
Previously (with the same 261), I used two stock-style mufflers (Walkers from NAPA) and they were quiet in the cab. The stock mufflers had no packing material and had a slotted/baffled straight-though pipe inside the round/cylindrical muffler.
However, from the rear of the pickup the sound was another matter. A local policeman who stopped me for speeding said he had to call me in when doing 55 in a 30 zone because the sound from the pipes made him think I was going to try to run him. I thanked him for the compliment and asked him if he would write that down to show my friends (he was just a kid and did not know that the ticket would disappear after a phone call).
