Sound Deadening and Engine Noise

[Gumby83]

Hey fellas, I have a project i've recently got on the road. Everything is great except the cab noise.

I sound deadened the hell out of the cab. Killmat on the floor/firewall with Siless foam ontop before jute carpet. It does make the cab much more quiet, but i got a serious resonance/drone around 2400rpm.

Going to rebuild the exhaust to try to isolate it but i want to know if there are any tips/tricks you guys might have to quiet the cab down.

Some facts about my truck are the truck was extremely rough when i got it, holes in the floors, rotted out cab corners etc. also doors don't close nice, they are abit out of alignment. The roof does not have sound deadening material. The engine is a built 370ci Turbo LS, i have a 4 inch downpipe that runs right next to the passenger firewall that has to stay. Any advice, would be much appreciated.