Isn't it a turbo so you wouldn't put on classic headers???? No easy better aftermarket exhaust manifolds available just better turbo's and exhaust pipe(s) after turbo.
He posted in the intake and exhaust, not the 6.5l section, so I'm going to assume he doesnt have a 6.5l and probably a gasser. If so, we need more details on the truck, whats been done, and future plans before anybody can make a suggestion. A 99 Sub could be 1500 or 2500, 5.7l, 7.4l, or 6.5l with a turbo. Need alot more info to give any response.