I have a stainless steel tank, sitting on a Fibreglass hull, and have used stainless screws and brackets around the tank for that reason, to prevent dissimilar metals.
The only dissimilar metals would be the metal in the fuel filter and fuel filter fittings, which are close to the tank. Cause of issue?
re "As for your tank, if you have positive and negative from the sender, it will work without an extra ground wire."
So, does that mean I don't need to worry about anything? I indeed have in and out cables from the sender... what about any connections to the filler cap etc...
This image has extra grounding for tank and filler cap: http://newboatbuilders.com/images/Fuel_Tank.jpg