Dean, if you can locate it, try using Copslip as the thread sealer.
It has little copper balls that crush as the threads mesh when the sending unit is installed. It helps with the electrical connection to the block.
Additionally, you can connect a 240 Ohm resistor ( 2W ) to see if the guage moves to the correct location. I recommend you do it first at the block and then at the meter. This will deal with the harness as a factor, if it works at the block, the harness is working.
If it goes beyond this, PM me and I will try to help.
