I saw a photo of a confirmed NOS 67 and the tube layout and the location of the reostat and the length of the arm the float is on are very different from my later production NOS and the aftermarket ones. I agree that all you mentioned will effect the graph now let me toss in one more for you...
The shape of the tank. If it were a pure rectangle one gallon added would raise the fuel level (just for example but not in reality) 1 inch the next gallon another 1 inch etc. but a-body tanks are tapered on the bottom so the fuel level will rise at a non linear rate till the level is above the tapered area, then it will raise lineally till it hits the carved out area for the spare tire.
I have seen the winding on aftermarket senders and they do seem to have the same number of winding per inch throughout the entire range and on at least one OE sender they were not.
ALSO (but I don't know this for sure) the gauge might have its own non linearity. I was building a resister box to prove that out but haven't finished it yet.