If you have not already done so I would direct you to this thread.
http://67-72chevytrucks.com/vboard/s...d.php?t=640615
The first two pics show a stock gauge with a bakelite resistor on the back which is 45 ohms. The OP of that thread has some of those in stock. They vary in resistance and are there to protect the gauge windings. The additional resistance is added via the sending unit in the tank by a variable resistor to ground, on the float arm, from zero (empty) to 90 (full). This is how the stock units work.
I would also direct you to the LS swap forum for additional information.
I have not measure the sender resistance in any of the in-tank pumps that I have changed but they all had four wires in the harness. I assume two for the pump and two for the sender. I'm thinking two of those were for grounds for the pump and sender, and one was the gauge sender wire and the other was for 12 volts to the pump. It seems to me that if you wired a 45 ohm resistor between the sender wire and ground, the gauge would show a half tank at empty. If you wire the resistor in series with the sender wire and the variable resistor to ground, then you would read the 45 ohms plus the 30 ohms with a full tank, but you'd still see the 45 ohms on the gauge for a half tank with it empty.
I know there is a way to make the old 0-30 ohm senders work by adding a resistor but I don't remember how they did it. I will have to do some research on this.