Mustang 5.0 overheats at 1,500 RPM+ but not at idle

I just got this car and don't know the service history. The 1990 Mustang 5.0 V8, 76,000 miles can idle in my driveway and never overheat, but when driving 1/3 mile at 15-25MPH to the store, I'm at redline if I don't let it coast periodically. I'm too scared to see how it acts at higher speed. I've replaced the water pump, a 180 degree thermostat, 16lbs pressure-relief radiator cap and a new engine drive belt. The hoses seem fine, and what I can see of the head gaskets, they look near new. Every bolt-on performance part is already on it including pulleys and a lopey cam (holley statemmax II is printed on the intake & AFR is on the heads). It does leak about 2 cups of coolant every drive. The upper coolant hose is hot, the lower is barely warm to the touch. The radiator fan is an on-off switch wired model. It's always on just to help. Where do I begin? I've heard of blocked water jackets. How do I check for that?