Are you sure your getting water flow to the radiator? I have a SBC installed in the back and the radiator is up front. Its using the stock Fiero radiator and coolant pipes.
I can hold it at 190 in some pretty high temps outside and thats with the stock Fiero radiator. I have a larger aluminum radiator that I will be installing at some point but right now the one I have is getting the job done.
IF you are getting water flow then its air flow that you need to work out. Perhaps you need some big air scoops on the side to flow some air into the radiator.