Magnum in 70 Dart, can't keep it cool.

Hi, I've got a 5.9 Magnum in a 70 Dart that won't keep it's cool. It runs cool enough in the winter (around 200+) but in the summer it climbs slowly to 225+. The temp climbs when the engine is under a load, or with the A/C on. If I remove the load (go downhill or pull over and idle) it cools off right away. It's the same at highway speed, if there's a load, it heats up. It has never boiled over but I can easily get it to climb to over 225.

It's got a Champion 3 row radiator, anti cav water pump, 17" electric fan with shroud, it's a mild, carburated, low compression engine. I've got the timing set at 8* idle with 25* total advance (but I've tried the timing all over the place with no effect on the temp). Today I put an aux pusher fan in front of the rad... no change. I'm running 25% coolant and water wetter... no change. I've tried several different thermostats (160, 180, 190)...no change. Notice in the picture that I removed the shroud flaps when I added the second fan. With or without flaps... no change.
I'd really like to get this thing to run in the normal temp range, but I'm at the end of my rope, I don't know where to go from here. I would put a clutch fan and shroud on it if I thought that would fix it but it gets hot at highway speeds as well.

Thanks. ](*,)