Stuart's right... It's probably the radiator.
However, there a few other things you can check.
1) Belt Tension - is the belt tight enough? Does the water pump spin nice, or is the belt slipping?
2) Water Pump - is the water pump working right? This could be a tired pump, and it could also be a pulley that's too big (seems there was a problem with new pumps having the wrong pulley on them a while back).
3) Do you have the right thermostat? A 216 should have a 140° or a 160° unit in it, depending upon the use (I think that the trucks had the 160° model, but I can't remember). If you've got one that opens up at a higher temp, that could also be your issue.
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I should probably mention that your radiator has that boilover tube on it, and a pressure cap won't do you much good...
