Detroit 12.7 issues, cracked exhaust center sections

So in December I bought a 2001 Freightliner Classic XL. It had a new inframe with paperwork from Detroit. I have been having problems with cracked exhaust center sections, cracked turbo housings, melts the plug out of the pipe where the pyrometer sensor would go if it was hooked up. Replace exhaust manifold with one from Pittsburgh power, and the matching turbo, neither has cracked yet in about 15,000 miles, but the truck smokes excessively, black smoke. It has new injectors, recent overhead, jakes rebuilt,...now that it is getting warmer it gets hot as well. On a long climb the fan comes on and stays on, but the water and oil temps keep climbing till the buzzer comes on. No cel lights on the dash, sometimes power surges also. There is so much soot in the exhaust when I start it up it rains black lol. I have spent a small fortune on this thing and I don't know what to try next...also I am only getting 4.5mpg. thanks for the help if you have any ideas

 

It is the stock 500hp tune from Detroit, it had a dura-lite cac on it when I bought so I hadn't checked it yet. Replaced boots, I'll have to get everything checked for leaks I guess.

 

Thanks guys, when I get home I'll do some checking and let everyone know what I find.