Electric Fan

I have a 1990 Silverado, the air conditioner works good as I have just put all new components and had her charged up, the problem is with older vehicles I have been told they must have air circulating in order to get it to work at its full potential, I have sat idleing and watch the temp rise, I am thinking of putting an elctric fan on it. This vehicle will be driven by my 16 year old son and I know he will sit idleing to talk with friends and I do not want it to run hot. I live in Georgia and it has been an extremly hot summer!!!! What brand and size fan would I need to keep the temp down or is it as simple as a themostate change? All advice is much appreciated!