DIfference is 5/16 flange vs 3/8 flange, Material & thickness of tubes / collectors...73 is the widest A body made so headers are not as BIG of an issue as on the 67 - 69 or even worse on the 64 -66......
I was answering the other posters question about your statement asking if the 67 and 73 differed and you were wrong.
That's your own quote above and it is wrong or misleading. You defined and limited the year, calling it the widest. You also make a distinction that 67-69 differ from a 73, they don't from a rail, basic inner apron standpoint.
I agree that it's in the year range of widest for the A's up to 76.
Like you, I should have worded my response more clearly.
Back at you counselor.
Have a good one and I don't care to carry this on further as it detracts from the OP's purpose.