12.7L detroit 60 series NOx

I am looking at a truck that had a recent rebuilt 12.7L detroit 500hp. Truck has been sitting for like a year. Will sitting cause any issues? It is a 99 kw.

Edit, disregard underlined.
Main thing I am curious about is the NOx rebuilt motor. I have heard the NOx epa certified rebuilds can cause a mpg dropped of about 1mpg. What does this NOx rebuild mean and is it reversible. What kind of mpg do you think I can expect to get with this motor. it's a 13 speed I will update with rear ratio asap.


EDIT: I just talked with the shop that did the overhaul. It seems that they did not add the NOx emissions. it is written on the plate on the motor where is says it was rebuilt that it is epa compliant and NOx reduction but it seems this is just the generic plate that is used and it was not actually done to epa standard. This sounds great and sounds like this is not weird at all right?

The truck also got a new clutch new drives haft and new steering something. Also all oil in transmission and rear end was changed. This sound like a good truck?

 

Thanks for the advice. So if it blows white smoke after just being turned on it's been "upgraded"? And either way this is not a Mechanical problem but can be fixed with a ecm programming? That shouldn't cost too much and I'll be able to find someone to do it?

What would this take to do, would it be a tuner and what would you recommend?

Is the 98 comparable to the 99 detriot 12.7?

 

Also the truck they are not sure if it is 580k miles or 1.58 million miles. Any ways to tell.

They are not sure on rear end ratio. What rpm would be good for this engine to be at for 65mph and 50mph in 13th gear? (I will be going flatbed otr.) They only have a road they say I can test drive to 50mph. Will be hopefully going to Chicago to see it.