My 65 is the hottest around the block

[Mo65]

My 65 can now be consired the hottest car on the block. I am begining to give up on my 65 due to overheating problems. Before it overheated during extreme hot days and at long idle stop and go traffic. But recently it has overheated chronically. Not to long ago it blew my new 4 row radidator. I took it to the shop and I was told that the car had a blow head gasket "for sure" and probably a cracked head. The heads were rebuilt and new gaskets were install. In addition, a new thermostat was installed, along with new water pump, tune up, (correct timing, etc. After the car came out of the shop the car continues to overheat at 3/4 of gage and reaches to almost H section. The mechanic is not sure why it overheats, and gave up on it, or gave after I paid him, I shall say.
I am tired of this problem, I don't know what to do. Some people tell me block might have sediment, and clogs radiator. I am not sure what it is, the only thing I know for sure that this problem was not as bad as it is now. If any of you guys experienced same problem I did, please let me know what you guys done to correct problem. Many people tell me this problem is normal with classic Mustang. Mustanf Gurus, I need your help. Thanks in advance.