I bought a new 3/8 sender for my 68 charger last year. I don't know if the old fuel gage worked or not because I bought the car and immediately started fixing it.
I was told that the new sending units had the Gm 0 to 90 ohm resister even though they were marketed as mopar sending units. On a charger forum there were members there that swapped out the resister wiper and float with their old one. Thats what I did just last week because I had the same problem you did last year. My dash and gages were restored by a dash company so I knew my gages were ok. I haven't got my car up and running yet so I don't know if what I did will work.
If you do a search in the charger forum you might find the link.
I know you said that your old unit didn't read any differently but I just wanted to give you additional info for you to think about.