If the radiator is a two row from a parts store....that is why....VERY WIMPY!
It is too small for you motor. I had the same problem when i was tight on money and went from my 3+ row radiator to a stock 2 row from autozone. Nevertheless...I had to go back to my 279.99 radiator from NPD.
Hope this helps
1970 Mustang Coupe. 302, Paxton Supercharger, Edelbrock Performer RPM heads (1.90i 1.60e), Barry Grant Mighty Demon carb, Performer RPM intake, MSD Pro Billet Distributor (w/ blaster 2 coil and 6A box), 8.8 inch - 3.73 gears, AOD tranny, X-pipe