In my opinion, the car is running lean. If the car is cooling at idle and slow speeds which exerts a huge demand for the cooling system to be right, and I have seen this many, perhaps a hundred times, and won't cool and the front end has not been altered (ie...air damn removed or modified) the carb is jetted too lean.
I am assuming that you have timing right as well.
No matter how much cooling power you have, a lean condition at cruise produces too much heat. Try going 12% richer and let us know how it cools then. I am sure this is the problem...I have fixed many this exact way.
BTW...changing the thermostat will only alter the initial opening points of the output from the block and plays absolutely no role in cooling ability. If the car runs 220 with a 180 stat, it will run 220 with a 160 stat under the same conditions.
Other subtle hints on overlean conditions include detonation or pinging, very hot and/or glowing headers, way too hot engine compartments, vapor lock etc. But, none of these may be present and the condition is still too lean.
A cursory way to determine a too lean condition is to read your plugs after a 10 minute cruise. White insulators with black pepper looking plugs are a tip off. Your plugs should look sandy brown or beige and maybe a little carbon around the outside metal base due to aromatic fuel ash.
For more on cooling...go to my cooling forum at www.inccn.net/techforum.htm
ConceptOne Pulleys and Brackets
Techical forum/links at www.inccn.net/techforum.htm
[This message has been edited by HOTRODSRJ (edited 04-24-2002).]