Q: I have a 1997 Ford 460cu rebuilt engine with new temperature sending units for gauge and computer, new water pump, new thermostat

My mechanic just finished installing a rebuilt long block 460ci in my 1997 Ford 1 ton. From the first test drive the temperature gauge continually fluctuated between 3/4 to almost fully overheating on the gauge. He hooked up his engine scanner, an extra gauge on the side and watched the dash gauge as well. All three moved together. The scanner showed 230 F as the peak temperature reached. The thermostat, water pump and radiator are brand new. When I first bought the truck the original engine did not have any problems with temperature changes. My mechanic is going to install a genuine Ford thermostat to see if that makes a difference. Is there a coolant passage that is partially blocked? How do you check for this kind of restriction, and how would this result in the constant temperature changes in the engine?

My F350 has 65000 miles.
My F350 has a manual transmission.