I.ve got a 75 CJ-5 with a sick 304.
I'll fill the radiator to about 1/2 inch below the fill neck bottom and the catch bottle to 3/4's. It will blow anti-freeze out the catch bottle vent, until the radiator is about half full. This will happen on the trail or just driving the 8 miles to work(temp 30 deg "Alaska"). I have tested the thermostat and it opens at the right temperature. My electric fan also runs alot more now, it even runs after a highway run on a cool day. The heater will only put out luke warm air.
I could use some help on this one, before I scratch all the hair off my head.
I don't think it's the radiator it is only two years old, and I had some work done to it last year and it check out OK. I flushed the block last summer during a rebuild, at which time I got a new cap, but as we all know things do go bad, I'll check it tomarrow at work.