If the previous engine blew a head gasket, then tehre is a possibility that the radiator may be defective in this car also. The usual reasons for head gasket failure are: (1) failure to change coolant, which can result in deposit formation and/ or acidic coolant damage to the radiator and (2) overheating due to a stuck thermostat, coolant loss, or other engine problem.
(1) You will need to use a scan tool to see what the computer is reading engine coolant temperature as. If it is above 220 degrees, then the computer should be commanding the fan relay on (this will be shown on the scan tool as fan status, or something similiar). Take a thermometer and directly measure the coolant temp in the radiator neck; if it is not close to what the computer is seeing, try replacing the coolant temp sensor.
If the coolant temp is over 220, and the computer is commanding the fans on, tehn you will ened to start troubleshooting the fan circuits to find the problem.
(2) Assuming teh coolant is reaching operating temperature (over 195F), you should tehn see the coolant begin to circulate in teh radiator when teh thermostat opens. If you do not, then you either have a thermostat stuck closed, or a faulty water pump. Also, make sure you ahve installed the belt correctly; a pump being turned backwards by incorrect belt routing will make a car overheat (yes, it is possible; we've all done it at one time or another!)
(3) If coolant is circulating, then check for a restricted radiator: using either a temp gun or your hand, check the tubes in teh core for sections that are cooler than others, which might indicate restricted flow.
(4) make sure that youa re not using pure water in the system, for reasons previously described.