I installed a universal sending unit in my tank and connect to my factory gauge,but when I wire it the needle goes way past full.so I took the sending unit out the tank to test it,so when I connected sending unit to the gauge everything worked just like it should,but when I put it back into the gas tank the needle just goes past full.now the unit came with a rubber gasket,and does the plate of the sending unit need to touching the gas tank with or without the rubber gasket? Please help

The gasket is meant as just that: a gasket, as in a seal, not an insulator, if that's what you're asking.
What happened to the instructions that came with the unit? There should have been directions how to set it up for various fuel tanks. There should also have been a simple schematic included, too.