Count me on the list of people with a replacement fuel tank sender unit that reads up to half (sometimes) on a full tank. Question: How does one measure the resistance of the fuel sender at various levels with it in the car? Disconnect the wire at the sending unit and measure across ground, or have the fuel system energized by the ignition and measure at the sending post to ground (that sounds like a short) or some other way. This is in a 69 B body so getting up to the pin on the dash is not possible. Thanks

if you measure at the post and then the body of the sending unit you should get the resistance measurement.
10 -73 or 78 as I recall.

69 B Body, I've been on the list for years.

You will still have to pull the sending unit out of the tank to measure resistence. How are you going to move the float arm through it's arc if you can't get to it?

Over time - full tank, measure a few times on the way down, get close to empty with a five gallon tank of gas in the trunk. Hopefully it is linear so I predict the zero point.

I think I also have a problem with a voltage regulator on the dash (that is a recent pop up). Since the amp and oil gauge has been replaced by aftermarket underneath, (all period correct sunpro), thinking of getting a fuel level gauge with the correct ohms measure full and empty , and installing it in the console out of sight.

You are going to be guessing at best. It's not that hard toi pull the sender and bench test it to be sure. You may just need to bend the float arm to get it to read correctly.