As stated by ERA535 having the proper radiator to start out is essential. Back that up with sufficient fan power and a shroud and you should be able to drive anywhere in any weather.
At speed the fans aren't as important but directed airflow is. In traffic or at low speed the fan and shroud are essential to provide enough airflow over the core so it can do it's job.
One of the most overlooked items when installing a new engine is putting in a new radiator. For what motors cost in these cars isn't it worth a couple hundred bucks to make sure your not going to cook it the first time you drive on a hot day?
To me not replacing a radiator when putting in a new motor is like using generic grocery store
oil during the engine break in period.
Heat and friction are the engines biggest enemies, why not give your engine the best chance at a long troublefree life.