I used thrush rattler mufflers on my 65'. They sounded great in my opinion not too loud at idle and lower rpms, though they were pretty loud when getting on it. megaphone tips made the truck louder of course too. New exhaust will be a open down pipe right off the turbo
Video i made of them
https://www.youtube.com/watch?v=DNyib3bO8GU&t=2s