Help with Cooling Issue Please

Let's start ,with the engine swap.Bigger engine,more hp.More hp,more heat.More heat=more demand on cooling system.I will guess ,it has a carb on it.If it does,(and if you live yearlong in FL...) I would recommend,a 160 or 170 stat.My guess,when youinstalled the thermostat,the recommended one,for a 5.9 Magnum was installed.They run hot,to pass tailpipe emissions.Start with that.No good,drop the rad off a a trustworthy shop,have em check it.If needed,rod itIn none of your response,you never stated,having checked the condition of the radiator.Done it,myself.