Radiator Size ????

I have a Demon with a 340 stroked to 408 CI sitting in my garage.
What would you think would be the minimum radiator capacity to keep running/cruising temperatures down?
It has what looks to be a six cylinder size aluminum 3 row radiator in it now. It is constantly getting hot when driving. This is an x race car, I assume the prior owner left the small radiator in it from when he drag raced it. I may need to get another radiator, but don't want to buy it twice, if that makes sense. What do you guys think? I have always ran big block cars and this is my first small block.
Car has an electric fan set to come on at 180*. No fan shroud, but an aluminum plate that does not look to me like it would work very efficiently.