First, I agree with Mike that dealing with a good radiator shop is a good idea unless your radiator is one of the new plastic tank models that crimp on top of the core. From what I've been told those are one time use, and not meant to be disassembled/re-assembled.

Regarding the need for a shroud, there's an excellent Tech article on cooling in the June 2013 issue of Street Rodder magazine. Don Armstrong of U.S. Radiator provided input for the article, and says,
Airflow management is crucial to low speed or idle temperature drop. The most important aspect of airflow management is a shroud and well-placed fan in order to utilize the full area of the radiator core. Fans are circular in design and radiators are rectangular so in order to utilize every square inch of the radiator a shroud is paramount whether the fan is mechanical or electric..... Without a shroud the fan is only pulling air through an area equal to the circumference of the fan, utilizing only 60% of the radiator's cooling capacity.
Get a shroud, Emmett.