First off, 200 would be OK if it was mine as long as it stayed there or went lower. My replacement aluminum radiator came with a 19 lb cap, I switched to a 14 lb, the 250 can handle that and the new heater core can handle it. Your heater core will be the determining factor on pressure.
Lack of air flow is why it's heating up at slower speeds, you need a bigger fan, more powerful fan, or a mechanical fan made for A/C cars or trucks. It takes a lot of air to cool the condenser and radiator. All A/C cars had different pulleys and fans to move the air and coolant faster.
