What you have now is a cross flow radiator. It might be too small but if it cools the car sufficiently at speed it should be able too cool it at idlle and low speed. The difference is mostly you have less airflow and less coolant flow at low speed and idle. I would focus on air flow first.
Better shroud and bigger fan. Might be all you need. Depending on what you have now a pulley ratio that turns the Waterpump and fan at a faster speed may also help. If you were having problems keeping it cool at speed as well as low speed and idle I would be more inclined to replace the radiator. This is assuming all things mechanically and tune up is correct. Just my opinion others here have good ideas as well.
