Having had habitual heating issues with my first 1957 Y-block Thunderbird I will share some of the do's & don't's I have discovered the hard way. I am assuming here that the basics have been examined (thermostat has been checked or checked and replaced, and that the radiator fluids have been checked and changed as needed, and that their are no constrictions in hoses, etc):
1. Use one of those stainless steel baffles that boasts it can reduce your operating temperature by correcting the spacing issue created by the water pump spacer on T-birds that were not on Ford passanger cars of the same era. The folks at CASCO prove those things actually create far worse heating issues if installed. From personal experience: THEY DO NOT WORK.
2. Believe that a new copper core radiator will solve the problem.
3. Believe that an electric fan will solve the issue; although, it may help.
1. Consider buying an aluminum replacement radiator from a Thunderbird parts dealer. They are often cheaper than the copper core replacements, and in the 25 years I have been messing around with & driving these cars, it is the best money I ever spent on one. My operating temperature legitimately dropped by 20 degrees immediately after I installed it with no further modification.
2. Consider a fan pulley that is 1" smaller in diamerter than what Ford installed originally. These are about 100 bucks, are billet aluminum, and are available through many T-bird parts dealers. The smaller the pulley, the faster the H2O pump impellor spins
= cooler operating temps.
3. Consider a better, more efficient H2O pump. There are two great replacements available that were engineered to reduce T-bird heating problems: 1. CASCO makes one with a greatly improved impellor. You can see the testing results on their web site. Or 2. John Mummert sells an aluminum H2O pump that has a modern ceramic seal, etc. Of the 2 the one from CASCO if more affordable.
If one of these options (or a combination of them) don't solve your heating problem then I would be concerned about bigger issues such as engine wear or other mechanical anomolies. If you recently rebuilt your Y-block, you can expect warmer than normal temps for about the first 7,000 miles. It should not be overheating however.