67 -69 vs 70 - 72 A bodies

Hi, can anyone tell me what is the difference between the 67 - 69 A body and the 70 - 72 models? I am trying to locate a set of 1- 3/4" headers and all that is available for my car [ a 69 ] Is 1- 5/8" and I notice there are 1- 3/4" headers available for the 70 -72 models. pardon my ignorance, here in Australia all our A bodies were pretty much identical in the engine bay/ steering form 67 to the late 70s