I have a 89 F350 with a 6.9 which has 5" stacks. They are way too loud, it is a standard cab and if I drive over 55 my ears are ringing. I found the stack inserts on the internet and was wondering if anyone has tried them and if so how much would they help? Here is the web link.
Quiet Muffler Insert for 5" Stacks - IOWA80.COM