OIL/WATER TEMP

[Chamberpains]

First off never listen to that mechanic again. Anyone that tells you to ignore your gauges doesn't give a damn about what they tell you.
Next you need to hook up some kind of scan tool, preferably Cummins Insite, to see what your sensors are actually reading.
After that if your gauges are reading way higher than the actual sensors are putting out, no big deal. Just figure out why your gauges are reading wrong, possibly bad gauges. But very unusual.
But if they're reading correctly...
You said you had a long block put in. Do you have any idea how much came with your long block? Thermostat, oil cooler, front gearcase? Some long blocks come very very very bare-bones. And you have to swap all the other stuff off the old engine and put on the new. Why I ask is because it's very easy to miss install a thermostat or damage the oil cooler putting it on. What is your normal running temperature at idle or on very light flat ground? There are a lot of unknowns with the description you've given us. More truck info and maybe some history of why you had the long block put in. Just in case you happen to be another poor soul with a very very bad EGR Delete and your ECM is causing all the problems. And possibly the reason you had to put a long Block in it to begin with.
At 230 degrees on the coolant you should get a check engine light. That is very suspect.