1970 Impala, 350

I just had Flowmaster 40's put on my 70 and the hanger came loose for the exhaust and the exhaust banged around a good bit. I took it back and they repaired that and said I was good to go. I told them that since that happened, I had been hearing a leak out of the engine where the headers bolt on, they checked and said the gasket needed to be replaced but that the hanger coming off and exhaust banigng around would not cause the gasket to have an issue. This engine was just rebuilt, its a new gasket, could the issue be from the hanger not being correctly attached and it banging around for a day or two? THANKS