The old downflow radiators don't work as well as the later crossflow ones do. You want the proper waterpump, as serpentine belt 5.0s rotate their waterpumps backward. The temp gauge sender is different between the early 'Stangs and the later fox 5.0's, though a later sender made the gauge read lower (cooler) in my '68. Other than that,, make sure your radiator is good, and that the thermostat and head gaskets are installed properly.
But first, you should hook up a mechanical temp gauge and verify the temp.