I'm getting tired of keeping a log on gas purchases and guessing what I've got left in the tank.
Original gage only registers when I'm about to run out-big help! I don't want to tear into the dash again to replace the gage. I thought I would hook up an after market gage. I have a new sending unit in the tank. So my question is how many ohms are there on full and how many on empty. I need this info to pick a gage that will work with my factory sending unit. Any help would be appreciated.

Sounds like a grounding issue. Is gauge grounded to dash? Is the tank grounded to the frame? The functionality of the installed guage can be tested before final installation. There are other threads on the subject. Have a look and I will see if I can find some pics.

What year and engine do you have? 67 & 68 400's have a return fuel line (vapor line) to the fuel tank.

Attached are some pics of original sending unit.

Here are pics of grounding connects to the tank.

17 years ago, after a dead empty fuel gage, I emptied the tank, replaced the sending unit and ALL associated wiring back around the tank, filled her up with gas.......still showing empty. One can only do so much.