I know I'm having cooling problems also but to melt the styrofoam sounds to me like the manifolds/headers are getting way too hot. And to me that would not be a cooling problem but a cylinder problem. Like
when your header pipe at certain cylinders turn cherry red! I have herd about it and only seen it once! And yes it could be a flow problem if there is any restriction in the cooling passages in the block, heads or radiator. That is if the water pump is the correct one to be using! Also a quote from the 69 shop manual in the cooling section is if you are running a steel 7 blade fan that the distance from the blades to the radiator should be between 3/4" to 1-1/4". Providing you do not have the cluch fan assembly! Doesn't say anything about how far in the fan should be in the shroud. I say the fan should be in at least half way or a little more. I need to get mine in at least a 1/2" more. I had it working okay with the 26" but I wanted to go back to factory 22" and went with a four core. It all doesn't mean sh-t! as long as the coolant isn't flowing well enough and not enough air flow thru radiator at idle! God Help Me!!