Ahhh... but NHRA does indeed list shipping weights in the classification guides. When you open the guide the page you are looking at has a white background and gives FACTORED HP to shipping weight calculations. If you pan to the right, the table is repeated for each engine/body style but this time listing the shipping weights and the background is grey. So for the '70 AMX the shipping weight is 3123 for the 360/290/280 and 3126 for the 390/325/315. You can calculate backwards from there and get the weight break listed in the left side table. The numbers in the HP/weight table are always done using the factored HP.
The rules for the GT classes is that for a body style that has multiple engines, you are supposed to use the shipping weight for the car "with the heaviest gasoline engine and appropriate transmission". So for John's Coronet, assuming it is a 2 door sedan, the heaviest engine, and therefore the heaviest shipping weight, is 3550 lbs. If you plug that shipping weight and the 405 HP for the 440 SixPak into my spreadsheet it looks like this:
