What is your exact configuration with regard to intercooler, fans, etc?
The stock radiator and cooling fan are EXCELLENT on the SRT's, and short of blockage issues, they wont have a problem cooling much of anything. Larger radiators wont solve your problem, and will likely create others, as they do not tend to last nearly as long due to design flaws. Adding more cores to an already sufficient radiator will simply decrease the overall flow rate.
I suspect that you are blocking the radiator with an intercooler, and if that is the case, the first place you want to look is increasing air-flow, and adding push or pull fans to your intercooler so that the radiator actually has decently cool airflow available.
Pictures of the front would help as well.