A flex fan will do nothing for you. You're not going to get any more airflow than what you're getting with that fixed 7-blade stock unit. Flex fans just put less drag on the engine when rpms increase.

My 440 is hooked up to an aftermarket aluminum rad and it never overheats. And it's an untouched mid 70's 440 with who knows how much crap built up in the block's cooling passages over the years, and it never even hints at wanting to overheat.

Remember, when the rpms go down, the water pump and fan are spun slower. Since you have that 7-blade unit, I suspect there are no more airflow gains to be had there. What water pump did you use when you put the motor together? Milodon makes a high-volume water pump, have you given that a shot? How about your t-stat? Some t-stats are just designed to flow much better than others.