My Dart has a big C body radiator in it and with the old worn out 340 in it, it could barely reach 180. I was wondering was was going to happen when the 408 is finished.
What many people overlook is that adding thickness to your radiator also increases the resistance to airflow.
When increasing the capacity of your radiator, you are best to add area before depth. Area does not restrict air flow like depth does. Max out your area first, then add depth as you need it.
Next, you need to look at the fan and shroud. The purpose of the shroud is to help the fan pull air through the radiator. You must keep the fan and shroud as tight of fit as possible for best efficiency. You should have enough clearance between the fan and shroud to accomodate engine vibration, but not too much radial clearance so the fan can pull the air better.
Next you look at FOOS (Not Chip Foose). It is an acronym for Fan Out Of Shroud. This is the amount of fore/aft overlap of the fan and shroud. The rule of thumb is 50% overlap of the fan and shroud. Meaning half of the fan is inside the shroud and half is outside. You can move it in and out a little, but if it is moved too much, it will also hurt the ability of the fan to pull the air through the radiator.
After that, you have to look and see how well "sealed" the radiator is. Are there any paths for air to go around the radiator and shroud? If there are, then try to seal them with foam or rubber. If the air is allowed to go around the radiator and not through it, it won't cool properly. Adding seals is more commonly found on new cars versus the older ones of our vintage.
Some people don't realize how important the shroud is. It is the most common overlooked/misunderstood part of the cooling system which has great effect on the performance. It helps keep the radiator running at maximum efficiency.
Also make sure that the water jackets in the block and head are clean and sludge/slag free and you have a good water pump.