I have seen the Bronco you are looking at when he took it to some of the local shows, its on the expensive side but he did a really nice job on the whole build. IIRC the interior is already dynamated, or was it just the top? , so I dont think you will do much to make it significantly quieter. A quieter muffler if the exhaust is loud will help some, bio diesel typically has less fuel knock and the station at Butler Market and Boyd acres has it at the pump. You might be able to play with timing a little and also the intake as the air filter setup he has on it creates alot of turbo noise so that will all help but not drastically.
Overall its one of the few Broncos I have seen around here that I would buy if I had the money in terms of being done right and not just lipstick on a pig.