Cummins ISM overheating

I have a 2006 international 9400i with a Cummins ISM. My concern is the overheating it's doing under a load. The truck can idle for 5 hours and just start getting heat to it, bobtailing it does not over heat but as soon as it gets hooked to a trailer I starts to overheat. It will get as hot as 220 and fan kick on on cool off to 210. Never gets over 220. Bobtailing it wont get over 180. Took the truck to Cummins and afraid they might be taking me a run for my money. They are wanting to crack open the engine bc they believe it is a blown gasket. Once they start into the engine they are committed and I dont want them to do that if not necessary. Well, I was wondering if a crack exhaust manifold could cause overheating? I do know that the exhaust manifold has 4 bad leaks and was wondering if this could cause the truck to overheat. I have noone to call nor talk to in town and need all the help I can get before I fork over 6k or more on something that might not be the issue. Please help before I go insane

 

We bought the truck drove it home, bobtailing never overheated. Next day put it on a load and it was showing it was overheating. When we took it to Cummins for diagnoisis a week ago the last time it registered as overheating was over 400 hours ago. They didn't see any other overheating registered on computer but cant say it hasnt before bc we dont know if anyone cleared computer before we bought it

 

When we took the truck to Cummins they stated that the last time the computer registered overheating was 447 hours ago from time we took truck to shop and was under a load at the time of overheating. The truck has not created only thing is fuel mileage gone to pits. I am just scared of letting them break into the head when they stated they are going off the dyno and the mechanic stated the following....

Run truck on dyno to check for low power & overheating. The dyno will not read horsepower. The engine overheated when loaded and cooled off when the load was removed. Could not run a bottle test on cooling system because truck does not have an overflow tube on it. Checked the cooling system with the exhaust gas detector & it tested positive holding the dector probe over the cooler reservoir with the cap off. The engine logged FC 2973 when on dyno

 

We only know of it overheating once. The very first time it overheated on a load we took load off and never pulled a load again. Bobtail only. When we took it to shop a week ago today Cummins said the last time the engine logged overheating was 400+ hours ago