According to the schematic in the back of the 08 Dyna service manual it looks like you need to add a resistor between the yellow/white wire and the black wire coming from the sender. The resistance ranges are.
Full 27.5 to 39.5 ohms
1/2 full 97 to 118 ohms
empty 240 to 260 ohms

Hope this helps - good luck, can't wait to see the final results.