The best thing you can do for drone is get the exhaust to not dump under the cab. The sound waves bounce up and down between the cab and the ground which causes that deafening drone sound. I would do some side exits or out the top or back of the truck somehow if you can. Also, ironically enough I’ve noticed trucks with dumps towards the ground kick up and blow a lot more dust around than top or side exits, especially at speed.
I think you would be fine with either size of tubing, the 3” will help the engine breath a bit more however and will be a deeper/louder than 2.5”.