[WADS56]

If you have a serpintene(sp?) belt you will need a reverse rotation pump.
You did not have cooling problems before because you had a smaller engine that made less heat. I am also guessing that everthing is ok like no coolent leaks, head gaskets sealing, etc... so i would try a flex fan or a real good electric fan.
The reason I am thinking that you need a better fan is because it is fine at highway speeds where there is lots of air flow. At slow speeds the fan may not be moving enough air through the radiater. I would try that first and if it still gets hot you may need to get a bigger radiater. I have the Autozone radiater in my 93 and it works great for a cheap price.