73' Duster fuel sending unit replacement
Not sure what the sending unit should read, but I see two issues here...both stemming from the original sending unit design
As you mentioned, it is not a linear signal
(Which makes sense since the tank is not "even")
I can't imagine that a aftermarket "cauge" would match that
BUT if it does, and the sender is bad, you probably will not find a good new one, since all the new ones are linear
Now, when you pull it out, I've found less then 1/2 is doable (on my 71 duster) but less is always better
Make sure you hold on to the stock locking ring too, the new ones are thinner and simply don't seal well