Running Hot on the Highway

Why did they recommend such a low pressure cap? I would also put a 180 degree thermostat in it, that will help it stay closer to the temp you want without such a large fluctuation. I put a new radiator in my Dart a couple of years ago, and had a problem similar to yours, even though it had the same number of cores as my oem leaker, the cores were MUCH smaller. Is it running lean at all at 65mph?